This all sounds great except for the one arbitrary mechanic that kicks in around 30 minutes that prevents you from applying more pressure like what happens for around 99% of gameplay.
That cool decision making about whether to replace a lost squad and risk getting wrecked by vehicles or build an AT gun and risk getting wrecked infantry? The basic mechanics for 99% of gameplay? Suddenly they disappear from minute 30-40 vs a pop-capped opponent.
I'm not sure if you're used to playing closely matched games. A player can still be winning without fully wiping his opponents units. Unfortunately in very closely matched games the player with the better pop-cap density is arbitrarily rewarded.
Consider a game where one player makes decisions 2% better than his opponent per minute of game. Ideally the game would be won around the 50 minutes mark by the player 2% better, but around 30 minutes into the game a player with a better pop-density composition can suddenly be rewarded with a 10% advantage against his opponent.
The first player in this scenario is arbitrarily punished for not being overwhelmingly better than his opponent despite being marginally better than his opponent.
I'm glad you brought that up since that's another aspect that's important. Pop cap determines what your opponent can ultimately field across the map due to it's naturally limiting nature. 4-5 infantry squads, machine gun or 2, At gun or 2. Some # of vehicles, etc. Knowing this you can summarize your opponents build and when fighting occurs you can mentally track what the field presence is like. If your opponent has 4 Riflemen and a squad of Rangers as his infantry and you force three of the riflemen back to base and you had to to retreat 1 of your Grens. Well now you have an advantage you can push for the next 30s-minute while your opponent is reinforcing and returning to action, if your opponents MG retreats now you have an open lane to make a free push with infantry, if this happens enough suddenly you're up effectively 200 MP in reinforce costs and you should be pressing your opponent incredibly hard. If popcap didn't exist and your opponent had 11 Riflemen and 6 MG's you push 3 of both off, that probably isn't going to matter because the map is flooded with units. You're correct in that Wipes aren't the only thing that occur, they're just the most blazingly obvious way to demonstrate massive MP advantage. Even in this situation you can attempt to snowball that small advantage if your opponent continues to make mistakes or if this were the early game if your opponents initial vry for territory failed you have an open lane to press the advantage via the denial of resources, the ability to create mines within your opponents territory, or green cover in advantageous positions, etc.
Also you have to realize the concept of the comeback mechanic here is to prevent the situation of "I lost x early, I lose the game now" which while it would reward the player who is immediately better. It's not very fun or engaging being on the absolute knives edge at all times for any small mistake and it denies a large part of the struggle of continuing that performance deeper into the game. If you're only "2%" better for the whole game as you say. It's too minor for it to be a winning advantage alone but it provides you more opportunities to grow that advantage and more chances for your opponent to screw up which would increase the advantage. Though the concept of "x%" advantage is pretty erroneous though since there are so many major and minor things that players can be good or bad at that you can't really quantify something so simply apart from what happens on the field.
As for the topic of unit diversity you're not quite understanding the core point of why you build units. Which is in order of desirability, 1. To push an advantage, 2. to fill holes in your roster, 3. To answer your opponent.
The reason for that is that you want to be proactive and not reactive with your builds if available. When you're building units proactively you're exploiting a flaw in your opponents build or their resources, in some cases this will end the game on the spot or put your opponent down so much that they are effectively dead, if this fails they can come back with a counter unit and stabilize the game sometimes. This only happens if the advantageous player failed in their proactive play, or they underestimated just how ahead they were. In most even games you should opt to build for generalist use. Having a generalist tank in an even matchup allows you to both push advantage somewhere offensively vs infantry, while also screening you from your opponents armor.
Lastly I'm not quite sure what you mean by pop-cap density. If your opponent has less things you should be winning somewhere by virtue of numbers, the bonus MP from having less total popcap isn't nearly as much manpower as you think. I don't remember the exact conversion but it's something like 2 MP per 1 pop cap which would be like 15 MP per minute for 1 less infantry unit which isn't going to turn a game, especially when that one unit missing isn't providing damage and isn't preventing damage to your units causing you to reinforce more and your opponent to reinforce less. It's entirely a mechanic to prevent steamrolling small advantages, not something to provide an advantage to a losing player.
If you've got anything else I'm ears but I think you're just vastly misunderstanding the concept here dude.