Medvedev is a factor, can't dent that.
As much as I mock Tsitsipas & Zverev , there is a reason many are waiting for them to step forward; they have had somewhat consistent results the last few years, making SF’s of Majors and in both cases coming within one set, heck, maybe 1 or 2 games or points of winning a Major. They've won Masters and YE champs and beaten the Big 3 multiple times. Yet now with them the question is "show me the money".
Outside of them , who do we really have to challenge the current big 2? Fritz just won a Masters, but there is still more a hope then an assurance that that will lead to more consistent play. Take Aslan Karatsev, made a huge splash last year reaching the SF at Oz, beaten Djokovic later that spring, etc. Now he's settled into that “dangerous” player in the 10-35 ranking slots including Hurkatz, Shapovalov, etc. Theim was the best bet besides Nadal for 3-4 years for Roland Garros but with him careening so far down who else are we going to bet will be making the finals of FO (barring the draw placing them in the same half) than Rafa and Novak. I guess what I'm saying, others can step up to make the FO finals but it's still more a hope than a done deal that will happen.
People around here get upset at Fiero for pointing out that the rigors of the circuit make the players inconsistent , but honestly the big 3 are simply freaks. Other than Jimmy Connors and Agassi, I can't think of anyone who's been competitive 13+ years in the highest levels of ATP tennis in the last 65 years of Open Tennis, and even then neither Connors nor Agassi won at the level of these 3 for so long...
Alcaraz seems to be different, but we won't know for sure until he lifts one of the those Major Trophys over his head, which is a heck of a lot of pressure to put on an 18 year old. I do think he's got "IT" , but time will tell.