Understand that my comment is not wholly about what they have or haven't done, but how the expectations on them affected them. Remember that I mentioned Zverev, and it's not like he's done nothing, and he's still only 24. But remember how long everyone was giving him such a hard time about not going deep in Majors? I thought USO 2017 was a particularly bad miss, since the bottom half of that draw was a complete land of opportunity. Dimitrov has some nice titles, but, as you say, he fell back away. (Oh, and on cue, he lost today.) I think we can agree that he lacks the fire in the belly. Peter Lundgren, who got Safin his 2nd Major, couldn't do it. And I thought Roger Rasheed who whip him into fighting shape, but nothing. Note that Dimitrov turns 30 in a fews days.
Interesting what you say about Roger's lack of fighting spirit. Obviously, it's relative, as you say, but he was the one, like Gatsy, for whom things "always came easy." And Nadal and Djokovic came up behind him, so they had a bar to hit. I can see why the Federer fans feel like there were a few left on the table when he was severely pressed, and his tennis, or the x-factor of grit didn't get him over the finish line. These are some that we can debate forever.
You very rightly mention Gasquet and Monfils as underachievers. Gasquet is the most forgotten one, on that score, if you ask me. He was pegged so early on for greatness, and I don't think it helped him. He's had a perfectly well-remunerated career, and he was once #7 in the world. Made the semi-finals of a few Majors. But think back to the 2005 RG round when he played Nadal and it was the battle of the teenagers. Such different fortunes.
But back to the youngsters who still have bright futures. I definitely subscribe to your Wild West theory as to what is coming next. There is no One next or even 2-3. Right now, it looks like a general scrum of some good stars coming up. It's impossible to know who will be the perpetual top 5 and GS winner, and who will be the Dimitrov/Gasquet of the group. We've been burned before. What I don't see is the transcendent star.
As far as Gasquet (and Monfils) is concerned, it is hard not to think that there's something to the idea that the French have a tendency to roll over. On the other hand, I've never been really impressed with Gasquet. I mean, he was a very good player but he just never looked elite, and with the competition there was just no way through the gauntlet. He didn't only have the Big Four ahead of him, but also guys like Ferrer, Berdych, and Tsonga. He reminds me a bit of contemporary guys like Bautista Agut or Carreno Busta; maybe I'm misremembering, but like those guys there was never a sense that Gasquet would break through to true elite status (even if, I think, Gasquet was a bit better than those two).
I do think we know some of the perennial top 5 guys of the new decade: Tsitsipas and Medvedev will be ensconced for years, and Zverev, Rublev, and one or two others will be frequent visitors at least. And I think at least some of these guys will win Slams, but I think the big unknown is the next "One," and I think we both agree that there's no clear candidate.
I vaguely remember asking this before, but I only followed tennis casually in the late 90s and early 00s so don't remember much of Rafa and Roger before stardom. At what point was it clear that they were "the One(s)?" I can't imagine it was before they were starting to win at least Masters--Roger in 2001 and Rafa in 2005. It was obviously very clear that Rafa would be great as soon as he started cleaning up in the 2005 clay season, even before Roland Garros. And Roger was on the radar for a few years before his ascendancy in 2004. But my point is, until it happens it remains unknown. Even when Roger won his first Slam in 2003, no one expected him to be as good as he has been. It wasn't really clear that he was an all-time great until he won his second or even third Slam. Same with Rafa, and Novak for that matter.
At the risk of another long post, I'm trying to single out a point. I think there are stages towards "the revelation of greatness." There's the stage the young guns are in now, where they're all candidates with some more likely than others. Then there's the stage when one makes the big breakthrough--not just a big title or even Slam, but sustained dominance. At that point there's the question of how dominant they can be.
I think the problem we're seeing with the young guys is that while several have flirted with that second stage--notably Zverev, Thiem, Medvedev, and Tsitsipas--none have taken it up another notch to the third stage: the #1 ranking and sustained dominance and/or multiple Slams.
I think I've talked about this before, but I kind of see it as if there are thresholds that are crossed on the way up. There's entering and sustaining a top 100 ranking. Then there's the jump to regular Slam seeding (top 30-40). And then another jump into and remaining with the top 20. Then a jump into the top 10, then top 5, then reaching number one. With each threshold, players peel away and stall out somewhere. Someone like Borna Coric rose quickly and very young, but then got stuck in the "Slam seed stage" (and barely that). Rublev and Berrettini are stuck in the top 10 stage, but not quite able to get into the top 5. Tsitsipas, Zverev, Thiem, and Medvedev have stalled out in the top 5 stage but haven't reached #1 (yet). And, of course, FAA and Shapo are stuck in the top 20 stage, recently joined by Sinner (who still has that fresh quality of not getting stuck anywhere, which makes him at least seem more promising than others).
But I think we kind of have to mentally dial back to the end of 2019, before covid started messing up the works. Since then the tour has just been weird. It seems to be readjusting this year, but there is still an impact. My point being, if we kind of "staple" the end of 2019 onto the beginning of 2021, some of those stallings don't look as bad. Or maybe I'm being too generous.