It’s so hard to predict with tennis, because not even 15 years ago, it was thought of to be unfathomable to win 20 slams, and now 3 guys have done it. Think of all the legends from the 1960s to the early 2000s that never came close to winning 20 slams. In 2002, Sampras’ 14 slams were thought to be an unbreakable record.
in theory, having 2 guys winning all the slams for the next decade+ as opposed to 3 would mean they’ll both have 25+ slams before they retire. Odds are still that neither of them will achieve those numbers because historically each generation has new challengers that will begin winning titles. In the case with Federer, Nadal, and Djokovic, they were so good that the generation behind them (players born in the 90s)never threatened to win slams (aside from Thiem and Medvedev who each won 1).
If Sincaraz can stave off their own generation + the next generation, then they will break the “Big 3” records, but there’s such a long road ahead before having that discussion