The big question for me is whether Alcaraz, TPTBS, and say Fonseca (if he turns out to be the real thing) can stay near the top for 13-15 years, big 3 MO, but highly unusual outside of them.
Yes, it is a big question: whether the Big Three will end up being historical anomalies (in terms of maintaining elite level past 35) or whether they established a new norm that other players will follow. We won't know for another 10+ years, when Sinner and Alcaraz enter their 30s, as they're the only current players that are vaguely close to comparable in terms of peak form and potential greatness (plus, hopefully, Fonzie).
I've talked about this a lot, but in the broader arc of tennis history, the "youthful game" of the 80s to early 00s are actually more anomalous than the Big Three's longevity. Pre-Open Era greats Laver, Rosewall, and Gonzales all were able to play at a very high level deep into their 30s, and Gonzales even into his 40s. Also, Andres Gimeno with his late Slam. Of course somewhat lesser greats like Hoad, Emerson, and Santana weren't....Emerson was still a top 10 player for the first few years of the Open Era, but his amateur tour dominance didn't translate at all, plus was already 32 at the beginning of the Open Era. Hoad was always erratic and 34 in 1968, and Santana was on a lower level than the others.
But the tour got younger, especially by the time you get to Borg and McEnroe - and the greats of the 80s-90s mostly started declining in their late 20s. Agassi was an exception, and then Federer's longevity was somewhat similar.
Anyhow, I think advancements in med tech are a huge factor, but also--as many have discussed--the Big Three played off each other, and worked hard to stay competitive. Like physical wellness in general, you can remain very fit after 25, but it gets harder and harder and requires more discipline. I think the Big Three just represent what happens when you combine stratospheric talent with working really, really hard.