If all you want is to get your troops to 100% drill, then it's relatively easy to rotate your generals between drilling armies. (Especially at 100% professionalism, they gain drill MUCH faster than they lose it.) Admittedly, I usually divide my army into crack troops (which get drilled to max) and chaff (which might get no drill at all, possibly because they're standing in CNs), and that helps to simplify things. (EDIT: A part of this is accepting that I will lose troops to sieges etc, and ensuring that sieges are done by subjects/chaff rather than elite troops.)
If you're trying to build professionalism via drill, I agree this would be hard in the lategame. But, surely you've already hit 100% before your army gets that big? In which case, it's reasonable to (re)build professionalism after slackening standards by mass-recruiting generals... so I don't see why drilling for late-game professionalism is important.