Oh, I completely I agree that a balanced force will beat out a force of pure infantry (especially if the balanced force is the one attacking). I remember seeing on AH’s website that something like 3 tanks + 5 inf offered the best chance for success on the first round, using only 30 IPCs.
My idea for a stacking limit - while still trying to keep very simple - was to limit the number of inf any country could have on the board at any given time, to their current production level (or some multiple of it). I was thinking of doing 2x your income (as being the max number of inf allowed) but that’s still a pretty big amount, IMO. I was also considering allowing Russia to have a few extras (it’s Russia, after all).
Anyway, at the beginning of your turn, if your income is, say, 45, and you had a total of 42 infantry on the board, you wouldn’t be allowed to buy more than 3 infantry. (But if you then lost territory, without losing infantry, so that your income was only 43 but you had 45 infantry on the board, you wouldn’t have to kill them off - however, at the beginning of your next turn, you wouldn’t be allowed to buy infantry).
Of course, if that limit is too low (for an income of 20 IPCs, you can only have 20 infantry), you can set it to whatever you want - 1.5x, 2x, 2.5x, etc. So if your income was 20, and you’d set it at 1.5x, you could have up to 30 infantry on the board.
You could even use a control marker with an “I” written on it, to keep track (on the production chart) of the number of infantry you’ve got …