It might not be worth it to upgrade mining assets if you have easy access to those resources relative to economic needs. On the map I'm currently playing, metal is a huge bottleneck in my (and the other majors') economy. On turn 34 nobody has managed to develop a metal deposit yet and the market price has gone up to 5 credits per unit. I'm going to have to upgrade the 3 recycling assets I've been able to secure in order to ramp up my economy at a reasonable pace, and if I can't find any metal deposits before the scrap runs out, I'll transition to using soil demetalization plants. If it remains this scarce, staying ahead in the amount resourced will be a huge military and economic advantage, and if I happen to not find a metal deposit and somebody else does, I'll need all the head-start I can get.
(I am not against allowing more developed assets to access larger, deeper reserves, though, that seems to make perfect sense.)
You scenario is the exception. Also a example of why upgrading is a bad idea:
The quicker you dig it up, the quicker you will run out. You will have somewhat more metal now, but once you factor in the costs for upgrades you will have less overall metal.
If you consume metal that fast, what will your economy do once there is no more metal to mine? Better to set up your economy for Metal scarcity. Also propably your army. The "Gas Powered Small Arms" are firepower wise a downgrade from Autmatic rifles. But they are really ammo efficient.
Also, your example would actually benefit from this. Once you think about it, aren't "make material out of Soil" plant just a realy pricy way to get really deep resources?
This is the final level - if no conventional mining upgrade will help to get anymore.
The change would simply add levels between those extremes.