Well, that's because I like it now just the way it is, even with Civs that have Cavalry as a unique unit. If Cavalry are so bad, what about Spain with the Conquistador?
No no. It's just fine as is. If anything, perhaps Cossacks ought to get their bonuses against Gunpowder units, to offset the fact that Cavalry now has a smaller role in the game. That would improve Cossacks to the point where they'd be truly very strong, but not Cavalry in general.
I think conquistadors are a great unit. Afterall you can beeline them since you don't need some other useless tech (rifling

Yeah a bonus against gunpowder and melee or just gunpowder units would be an acceptable solution. I'm more concerned about Russia so I don't mind if only cossacks are improved.