These are all good points, guys. The Khuzait heavy horseman armor seems pretty heavy to me, or at least looks that way. So it's hard to see how some foot archer armor could be much better than that. But looks can be deceiving sometimes, and maybe the armor stats are a decent amount better. I wish there were a way, like in Warband, to see what armor a soldier is wearing. I mean I can tell what kind of armor a soldier is wearing by comparing it to the graphics of other armor via the inventory cheat menu, but who knows if that armor the soldiers wear is the same stats as that armor, or if it's just the same graphic but different stats.
In any case, yes, I see some of the scenarios where foot archers could be better than horse archers. Those reasons just seem kind of underwhelming to me. Like the accuracy thing might be a little better, but probably some negligible amount. Being a little more athletic in sieges could be nice, but the upside of having a horse during field battles is much better than this. The upgrade cost doesn't seem to matter quite as much when you're making tons of money in the late game, but it still seems to be one of the more compelling reasons as to why cavalry archers should be more powerful. Plus you have the fact that only a few civs get good cavalry archers, and that's like their thing, so it makes sense why they should be more powerful. Not everyone can access this stuff.
Still I feel like a Khuzait Heavy Horseman is probably the best unit in the game right now. An equal amount kills any infantry and any foot archer. And while it has a weakness against melee cavalry, that weakness isn't super glaring, given that the Khuzait Heavy Horseman has heavy armor and good melee capability.
But I guess there has to be one unit in the game that performs better in a wider range of scenarios than the rest. And it makes sense why it would be a heavily armored cavalry archer with good melee ability. I just think that they should cost a bit more, not just to upgrade, but also for upkeep somehow. I like Gerileone's idea for this.