On the other hand, I wouldn't write Alcaraz off from such greatness. He has the complete game to do that. But if he "only" wins, say, 6-9, it is still more than any player other than the Big Three since Agassi. Remember that in the Open Era, only a dozen players have won 6+ Slams (not counting cross-era players like Laver, Rosewall, and Newcombe): Connors, Borg, McEnroe, Lendl, Wilander, Edberg, Becker, Agassi, Sampras, Federer, Nadal, Djokovic. Becoming the 13th player to do so is enough to earn the appellation "all-time great," imo - and I think that's already a lot to expect from any player.
So my suggestion would be to re-calibrate to that, to ask the question: "Who will be the next player to win 6+ Slams and join the other all-time greats?" Alcaraz is still the most likely player, followed by Sinner and then a gap to Rune. If one of them reaches that mark before, say, turning 26, we can start dreaming bigger. But getting to 6 Slams is already quite an accomplishment, despite how easy the Big Three made it seem.