As I said, the innovation of the Medieval Knight was "charging full speed ahead with crouched lances". "Charging" cavalry, in that sense, was not a thing before. Byzantine Cataphracts fought like all known cavalry of antiquity: they didn't crash into enemy formations, they slowed down and attacked like infantry trying to maintain formation.

  The cavalry charge became a lost art, because people learned how to counter it

I don't know, mounted knights charging where dominant from the mid 1000s until around Agincourt (1415), that's a long time to find a specific counter. What happened imho, is that armies got big again to the point where the charges wouldn't scale anymore. Armies started resembling more the classical period: the landschneckts, and specially the Spanish Tercios resemble Alexander's armies a lot.

And as for being a lost art, the art was really in the breeding and teaching horses to trample through a formation. I imagine it would have been a useful resource in later periods, it's not as there where no <8k unit battles anymore. It's just that since that wasn't the META for the important battles nobody could justify maintaining such an expensive skill.