@JackGates
I think there are multiple factors here:
1) Big 3 were improbably talented
2) Gen Useless was improbably worthless
3) Court surfaces were homogenized so if you're better than someone in one place you're better than them pretty much everywhere, allowing you to build a huge mental and tactical edge against players worse than you
4) Huge increase in prize and endorsement money around the early 00s making older players more likely to stick around
5) Increase in money means that top players have exponentially more resources to improve their game and look after their health compared to young players
6) Surface homogenization started happening around 2002, so then next few years of youngsters still grew up in an environment where they had been exposed to a variety of play styles and a complete set of skills instead of just baseline bashing
7) Racquet and string technology changes stabilized, preventing older players from being forced out by new tech
8) Less and less opportunities to get BO5 experience outside of slams as now pretty much everything is BO3.
9) Young players get an early exposure to fame and money now which can derail them (see: Tomic, Kyrgios)
10) Young players marinate in the modern weird low-work-ethic, low-struggle, constant-social-media environment which might affect them negatively
No one in the next gen has shown big 3 levels of promise YET, but at least we now have a few prospects with potential. For 8-10 years there was literally no one on the horizon who could even hope to be a top tier player. A guy like Cilic would have probably been at least a 2-4 slam winner in a different era but even he belongs to the previous gen if I'm not mistaken (same age as del potro so the end of the Nadal, djokovic, murray, del potro group).
In other sports, young players are still breaking through all the time, even though training techniques have allowed older players to stick around longer than in the past (e.g. Tom Brady). In hockey, amazing young talents are breaking through every year, but in tennis we had a weird 10 year gap.
Is there a popularity issue leading to less high quality athletes entering tennis? Seems unlikely as viewership has been increasing every decade as a consequence of greater media reach.
