Zverev's problem at Slams seems to be mental, it is as if he's daunted by the prospect of a 5 setter alone. Take the match against Fritz, blew it, saved by the dark, then came back when it's essentially a 2 setters and destroyed him.
Kyrgios has made no secret of the fact that he doesn't enjoy tennis all that much and would prefer to do something else. Can only motivate himself against the top guys, which is a shame given his undeniable talent, but that's just life, it happens.
The likes of Rublev, Chung, Tsitsipas, Shapovalov seem to be much more put-together. It's hard to tell whether they are ready to challenge once the old guys are gone or the Raonic/Dimitrov bunch will takeover for a while but one thing for sure, it'll be a long long time before we see anyone with the class of Federer, Nadal, Djokovic again.