5 of the 8 in QFs are "NextGen" players, which in this case I'm defining as "anyone who counts as NextGen if Medvedev does."
I think they're doing quite alright.
Right. The three players the OP points out - Rublev, Zverev, and Medvedev - are all best suited to hard courts (Medvedev) or some combination of hard courts and clay (Zverev and Rublev).
On the other hand, the players aged 25 or younger who are in the last eight at Wimbledon all have games best suited to some combination of grass and clay (Berrettini) or grass and hard (Shapovalov, Auger-Aliassime, Hurkacz), with the sole possible exception of Khachanov, who benefitted from Tsitsipas's early exit.
If there were more tournaments on grass, it might well be that I'd say of all of the first four players through that, like Medvedev on hard, grass is definitively their best surface. I can certainly imagine thinking that of Shapovalov and Hurkacz, but possibly Berrettini and Auger-Aliassime, too.