I have to agree somewhat about your "admin" comment. Basically, every turn you have to figure out where some replacements should go and where to build new units. In addition, due to the readiness loss when transferring outside the command tree, it is best to assign production to an HQ. But, each turn you have to select an new HQ and transfer, etc. And, if you forget, you find an HQ with too many rifle, and difficulty getting rid of them. This is a bit tedious, but at the same time part of what is good about the game as you get to build your army your way.
On the first half of your comment, while I find the "end game" something of a letdown, I hate to admit it, but this is pretty much what war (from start to end) between nations boils down to. The letdown comes from having crushed the front and the other side tries to rebuild, but the units are weaker because their readiness and experience are low, and you keep pushing them back. But, this can take a long time because the distances might be fairly far (in large random games) and since they make a line, you have to attack to get through. So, it will take several turns to end it, which all seem the same, and are grinding to an inevitable conclusion. But, this is not far from reality. Think of the eastern front. After July '43 (and some may even say January '42) the Germans were essentially done, but it took another 1.75 years to get to Berlin and finish it. And if it wasn't for Hitler's stupidity, it might have taken as much as three years. The same in the Pacific. By early '44, the Japanese were hopelessly over matched in terms of equipment and the outcome was pretty much inevitable. The only question being in the U.S. could take the losses.
In the end, the grand encirclements and whatnot are really just a part of wearing down the opponent to the point that they can't fight.
I guess the best way to avoid this grind and letdown is play a scenario that has well defined objectives and endpoint. That is a nice feature of this game, you can play either type of game. Especially, with an economy. Some other games, like TOAW don't have this feature.