I don’t think it will be a free for all, I don’t see any of the current players being anywhere near the level of Alcaraz or Sinner.
Barring a big shock, Sinner is going to win the US open. And going forward it’s likely going to be one of those two, and in the rare event that neither of them do you might get Djokovic pick up one more or Medvedev possibly.
I don’t really ever recall a time when Men’s tennis was a free for all barring say the individual grand slam (in the 90s, Sampras dominated but never came anywhere near winning the French open). There’s always a slight transitional phase that might last a year but that’s it, 80s went from Borg to McEnroe to Lendl, and along with Lendl, Becker, Edberg and Wilander quite seamlessly…
Alcaraz has already proven that he can win majors on all three surfaces (though if you want to be technical, the hard court at Flushing meadows is different to that at Melbourne in terms of bounce…but that’s not enough to say that they are different surfaces)
It would be good if two or three players managed to get to the level of Alcaraz and Sinner (although that implies that those two are at the same level and they aren’t) but I don’t see any young player emerging….although you’ve had plenty of young guys look like they might emerge but never make it - Zverev, Shapovalov, Auger-Aliassime, Tiafoe etc