That's why I chuckle a bit when some complain how trying to slow down the grass or HC has "ruined" the game, with todays rackets/strings Karlovic/Isner would be perennial SF's , Finalists at Wimbledon.
To your earlier point on Medvedev, he's still what I call on the very tall side at 6'-6" and moves as well anyone that height since Marat Safin (6'-5"). Del Potro unfortunately was/is injury prone. I've never seen someone that tall with this type of game, whose willing to grindout long extended rallies and run like a gazelle on court; of course the Grumpy Russian's ace in the hole (no pun intended) is his serve, which can be dominating.
Can he keep it up with his height is a looming question I have. As a Nadal fan, I'm wary of dismissing others chances of longevity; ie they can't sustain it (Nadal's naysayers date back practically to when he started playing) but I do find Medvedev to be a fascinating unique ATP player in that sense.
As to Medvedev and clay, I think it's safe to say it's all mental. Loathes the surface with a passion.