If USSR allows the entire wehrmach to deploy on the border in may/june 40 without countering it in some way, of course they die, and of course they get what they deserve.
But russia should not allow this. Assuming russia is doing this is like assuming the french would deploy on the spanish border, you don't realy need a brilliant german to defeat such a (lack of) defence. If Germany doesn't take Poland by nov/dec it's standard tactic for russia to send some troops to the border, both to weaken the german hand (a bad diceroll in poland can easily mean russia can break the pack because of polish manouvering placing ZOC on germans), and to cover the eventuality that germany gets any ideas.
If germany builds up more than strictly neccesary in Poland, russia must counter with a smaller, preferably mobile force.
Such a force can easily be used to throw off the entire german game plan (let germany forsake other fronts and build up in jan/feb + mar/apr, then move say 5 corps, rail 1, and rebase in 3 air, for about 10 extra garrision in a single impulse. Such a force doesn't need to be deployed ON the border (where it is vulnerable to german DOW after a quick german reinforcement + aligning minors surge), but can be placed in safety 5-6 hexes away from the border, backed by a rear HQ
Remember, germany basically needs a 4-1 garrission to attack, so russia can very quickly block any chance germany has at attacking, if it so chooses. The trick is to keep a minimum reserve of 1/3-1/4 of the strenght germany keeps at/near the border. Add in rebased air, and the germans might very quickly be unable to break the pact. The same force can alternatively defend/screen the motherland if stuffing the border seems to risky. But threthening to stuff the border SHOULD discourage germany from any sort of big buildup, because russia CAN throw in units faster than germany, at a much lower cost in lost opportunity elsewhere. A big buildup in Poland is an invitation to a wasted and idle summer 40!
OK, but let's assume russaia can't or won't stuff the border (after all, there IS a significant risk..)
In a 40 barbarossa, russia can afford to let the nothern front stand completely undefended (or very lightly defended, say a stack of reserves in Minsk plus 2 mobile units that stay outside air range). Germany will still have to send a HQ and at least 5-6 units north, or else russia can move some units north and cheaply shut down the northen front (because with less than 6-7 units, germany simply lacks the volume to kill a single doublestack + cover it's flanks&supply)
Geramany also needs to keep CW honest, and that should tie up at lest 6-7 other units of some quality (2 in denmark and 4 in France/Belgium?). So germany is down about 12 corps and 1 HQ. That's almost the entire german starting army, especially if a few losses in Poland/France are counted in.
So the german offence consists of 2 real HQ, the german reserves, any reinforcements that have been built and arrived, and a few rumanians (HQ antoniescu is a reserve!). The finns are easily countered in the north by BEF, and in the south by 2 corps (reserves?) in Leningrad. Some will slip through Kareliaand join up with the northern offence, sure, but will not reach anything very vulnerable anytime very soon.
The main german force will not be very impressive, maybe 2 HQ, 12-14 real units and another 6-8 weaker losstakers/screening units, plus probably 5-6 bombers and 4-5 FTR. That's ALMOST 2 killstacks, but assumes the ENTIRE available army was ready on the border at the time of DOW. More likely, another 5-10 corps and significant air will have to rail in and be available only for the second turn, leaving just 1 killstack in the south, too.
But let's assume a german strenght of about 20 corps, russia should at least have a force of 6-7 corps + 1 HQ plus some air being in range of flying in by the time of the attack. There should also be some reinforcements that just arrived, say 3 corps arriving in cities along/behind the Dniepr. Russia should be able to deploy a screen vs Rumania (assuming germany doesn't start there) with the mobile forces. The screen is anchored in Odessa (which is empty but gets at least 1 reserve!). Other reserves go in Leningrad, Minsk and in Dnep/Kiev area, + possibly 1 or 2 in key cities on the northern route.
Russia will possibly deploy a single speedbump in the south, but not neccesarily. with 10 mobile corps russia can hold the rumanian border 1-2 impulses, and mount a mobile ZOC-defence across Ukraine. Russia will deploy strong MECH/ARM stacks just outside axis FTR/Stuka cover, and rebase it's own FTR to cover the stacks against the few axis longrange bombers. At the same time it might use the HQ to reorg 3 reserves, but then again, maybe it's better spent trying to spend any key units that are flipped/retreated. Anyays, this defence should see very few losses the firsr ~3 impulses, until russia withdraws behind the dniepr and makes a stand. Starting impulse 4 (or 5?), Germany can start attacking over dniepr. But germany lacks enough units to make more than 2 attacks/impulse, and russia still has plenty of units to envelop/ZOC any breach across the dnipr (at least if it reverts to ZOC defence elswhere). Exactly where things end up is difficult to tell, but realistically, germany will be barely across a crumbling Dniepr line in the south, and probably near the dniepr in the north too (but with a weak force). 4-5 factories should be railed out, and probably Odessa and maybe 1 more lost. Russia recieves reinforcements, can reorg 4-5 corps that were railed in (from wherever they were) and bulds for well more than 20 BP, probably half of that MIL.
So now, russia should have about 5 corps that railed in last turn, possibly a few that walked in (say 1), a few surviving reserves (say 3), a few MIL that arrived reasonably nearby (say 2), reinforcements (say 4), and maybe some corps that were spiralled (say 1) = 16 extra corps available for service, probably including Zhukov.
On the german side we assume very few losses(say 1), and that they didn't have the unfortune to mess up in Minsk/Odessa/other big battle (1 flopped attack could easily stop the whole offensive way short of what is assumed above. some units (say 4) are flipped behind the front, and the reinforcements start waaaaay behind the battle. In the north, a few units were split off to take the baltics/screen Leningrad, but this may have been compensated by a few finns sqeezing through. So in the north the unit count is about 7 units (mostly unopposed by real forces), and in the south it's about 10 units + 5 weak units, with another ~5 in the rear.
Then you add about 8-10 surviving russians to the 16 new arrivals, russia has a sligth advantage in numbers, but the axis will even this out when it gets the rearguard into the battle. Still, russia SHOULD be able to survive over the summer without a major collapse, and the massive production should stop any axis dreams over the long winter turns. Russia will churn out units at an impressive speed, and without the help of armor and air in winter, russia will hardly take losses, while the german position will be very, very stretched. I think.
By asleep at the table I mean "not doing anything to tie up germans/do damage" while all them germans are in the east fighting.
Allies can surely loose france very early from a france first strategy, due to no fault (other than poor luck) on the allied side.
But if they proceed to do nothing while the axis redeploys east and launches a '40 barbarossa they are indeed asleep!!
Earlier than may/june a barbarossa will not get very far simply due to weather. During summer, some defence has to be mounted, that is why russia needs a small mobile reserve IF the germans start massing at the border. A mobile reserve doesn't need to be very large, just enough to slow the advance down a bit through ZOC defence, a few hero cities, etc.
In early 40, germany will simply lack the volume (both in air, land and HQs) to kill enough russian units. So if weather or a screen of mobile units can slow the germans down enough until reserves/production/bad weather arrives, the russian army can very quickly grow equally large as the german one. This effectively prevents germany from inflicting havy casualties or gaining huge chunks of land from that point on (perhaps until summer '42, where pure volume on both sides again allows sufficient loss rates to mount, even on a stalemated front).
But without the continental base in France, how are the Allies supposed to tie up enough German units? The CW, even if it cranks out amphibs from the start of the game, won't have any available until 1941, which severely limits its invasion capability even for tactical raids, as does the lack of CW marines until 1941. Paratroopers by themselves (which are expensive and won't appear until after summer 1940, anyway) aren't really up to the task.
This lack of force pool options and of sea lift constrains the Western Allies. CW has only divisions to invade with, in effect, until 1941 (since the PARAS won't arrive until the '40 summer season is over, or in J/A if you built them ahead in S/O 39). That might do against Sardinia or Iraq/Persia on a surprise impulse, but it won't anywhere else.
Aside from pinprick strategic air raids (the strat bombers suffering from the same time constraints as PARAs), there isn't much the CW can do against Germany in 1940 once France falls. Even if it has retained a beachhead somewhere, all the Germans have to do is screen it - the CW doesn't have the ability to get ashore in force until the amphs and marines arrive. Similar constraints operate against the war vs. Italy (except that the CW can probably assemble adequate forces, even in 1940, to knock out Italy's aligned minors in Africa).
Letting Italy get at the Middle East may be the result of an Allied error of judgement, but it might also be the result of the same military constraints the Allies operate under in '39-'40 - whenever I play Italy I always set up strongly against Egypt, and in RL games I almost never fail to secure it (either on my own or with some German help), and it's usually because the CW doesn't have enough units in the first year of the war to meet all its commitments, and a fair chunk of those units get destroyed in France (the critical theatre). When there's 1-2 TERR and maybe a real unit defending all of Egypt, even the inept Italian army, when deployed in force, can usually manage to overcome them.
Finally, as lavisj points out, the Germans aren't likely to launch an offensive against the USSR until the summer in any case - Poland (usually) needs finishing off first, and they will want to deny the USSR the delaying options that bad weather opens up.
I don't think it's impossible to defend against a 1940 Barb (in the unlikely event that the Axis get the chance to launch one with France already defeated), but it is very difficult indeed, and it will fall almost entirely on the USSR's shoulders. A 1941 Barb is much better from the Allied point of view not because the balance of forces on the Eastern front is better (in most cases it's worse) but because there are enough Russian bodies to throw in front of the advancing panzers to give the factories time to rail out and the CW will, unless it was particularly negligent, have a couple of amphibs to use in tactical raids (or invasions of Sicily, Greece, or other great staging areas), an army to raid with, and a small but growing armada of strat bombers to menace Axis production and oil.