Even in a Fortress Europe for Germany, or a Fortress Moscow for Russia, there are still other units being bought besides INF. Sure, once things get REALLY bad for either nation, they go to 100% INF. But until then ART and ARM are almost always in the mix, and the occasional FIG is not uncommon.
Once you go 100% INF in most games, that is an indication that you are losing. There are exceptions of course (for example when Germany and/or Russia have spent their forward INF but have saved their punch forces and need to replenish their INF supply for subsequent combat) but even then the INF builds are usually to restore force balance… to get enough INF to accompany those ART/ARM/FIGs to the front for battle.
The thing with major INF builds is simple…
If you are on the OFFENSIVE and are building massed INF, then you already have lots of other forces (ART/ARM/AF) to use, you just need “fodder” to replace losses, and/or to secure the homeland.
If you are on the defensive, INF is nice bulk, but still is best when backed by some ARM and FIGs to increase your punch on defense. A pure INF defense is a losing strat that can be strafed into oblivion.
So really, in a high grade game, there is no reason for limits on units. A good player creates their own balance of forces… otherwise they are not really a good player are they?