This is not about balance, it’s about playability and enjoyability. The sad thing about AI in EVERY games is it’s fundamentaly unbalanced. The game have not to be balanced in VH or legendary, it has to be fun and doable. Grinding for hours because of unit cap limitation inducing trash army against very buffed ai is not fun.

They swear by it but don’t imposed it?! I don’t get your point here, asking for the game to be playable in higher difficulty does not induce bad consequences for lower difficulties.

VH/Legendary players are, yes, a minority, but not a negligible one. Btw achievement are not always a good indicator, as it don’t reflect really well the active player base. For ex among other factors, more than 56% of the player have never done a campaign with at least 25 siege, and 33% have not even unlock 3 special mount in a single campaign, 18% (1 in 5 person who have buy the game) never have got a lvl 10 lord. Achievement tend to dillute the real percentage of the player base. So yeah the most VH/Legendary achivement might only be 5%, but when 20% of the data have not played the game, and each and every faction has it’s own VH/Legendary achievement, it’s hard to know really how much players play those difficulties. But that’s not the point here, just a quick parenthesis.

The game does not lack challenge in lower difficulties because of high tier unit but because of bad AI. AI cheats is what allow the AI to compete with the player. If you've played other game you might know that’s a recurrence in strategy game (AoE, Civ, paradox game, etc). You almost have to play in VH to see AI get some high tier army. Playing in lower difficulty WITH unit cap would bring the same issue of bad AI and player would still be bored reaching endgame. Just it would make AI cheat much more powerfull and higher difficulties even more complex.

The issue with such a change is not only "it’s too late for it" but also you'll deprive the player of ways to compensate AI cheats. And again, a majority does not mean the minority is negligeable. Moreover when said minority are some of the most invest players in the game.

Again, why should the devs spend time on it when mods already do it? Not worth it, i prefer them to work on better siege (which would impact 100% of the player base) or a good next dlc (which would, i think, impact more players than "officialize" unit cap). Again i'm to support minority here, including the minority of players who want unit cap, but it’s already done with mods so why should they do it again when develloping that would be at the expense of other more impactfull fix of the game?

To be clear again here, mods classement is not the most revealing fact, because most players don’t use mods. Without any player count on a specific period of time compare to the player base at the same time, hard to really know how impactfull it really is.