I see most are assuming Sinner and Alcatraz will keep dominating majors in the short and middle term, as they did this year. It is possible? For sure it is. It is likely? That's debatable to say the least. Is it a given? Hell no.
Just about anything is debatable (See; "Internet, but especially Twitter"). But let's also not undersell just how good Sinner and Alcaraz have been. Over the last three years (2022-24), Alcaraz has won more Slams than Andy Murray did for his entire career, and two-thirds the totals of Edberg and Becker. His overall performance level, year to year, is pretty much comparable with peak Becker/Edberg...and he just turned 21. I suppose there's always the ghost of Jim Courier, who pretty much played like an ATG for three years then dropped to being a tier 2 (Berdych) type for a couple years, then a tier 3 guy for a few years more. But he was already Alcaraz's current age when he started his three-year run of almost-greatness, and his decline is unusual for a player that performed as well as he did in 1991-93.
As good as Alcaraz has been for three years, Sinner has been even better in 2024. By my statistical nerdery, his overall performance level this year is better than any season by Agassi, Edberg, and Becker - and the season isn't over.
To put some numbers to it, I use a stat I call "Season Dominance," which basically combines three other made up stats: One that measures good results at every tournament; one that measures shares of titles won; and a third that is a percentage stat of quality of play (that is, how well they did as a percentage of what they played in).
For reference sake, a truly great season is about 100 SD - and ATGs tend to have multiple seasons above 100, with 150 being roughly top 25 in the Open Era. Players like Zverev, Medvedev, Wawrinka, Thiem, Del Potro, Roddick, Hewitt, Safin, and Kuerten peak out in the 70-100 range. Ferrer's best was 68, Davydenko 56, Tsonga 38, Berdych 33, Gasquet 26.
Sinner is at 137 going into Shanghai - which is the same as Roger's 2009, which was his 6th best season by SD. Here are the players who have 137+ SD seasons, in order of highest best season:
Djokovic: 233, 201, 185, 161, 155, 145, 142, 142
Federer: 221, 187, 180, 177, 157, 137
Laver: 216, 138
McEnroe: 215, 150
Lendl: 186, 163, 155, 148, 143
Connors: 184, 145, 143, 140
Borg: 179, 178, 154
Nadal: 178, 173, 158, 138
Murray: 170
Sampras: 162, 146
Wilander: 146
Vilas: 146
Nastase: 145
So by my system, Sinner is the 14th player in the Open Era to have a 137 or higher SD. His 2024 season is already 41st best in the Open Era, and almost certainly going to pass a bunch of those above and reach the top 30 seasons, with a good chance of reaching 150+, which is top 25 in the Open Era.
(And yes: this sort of stat is debatable. I'm sure some will dislike the fact that Andy Murray's 2016 season is better than any by Sampras, or that Wilander's 1988 is so relatively low...but the system weighs all tournaments, all wins - and yes, does heavily weigh Slams, but tries to take into account the fluctuating nature of "Slam prominence").
While this is just one system of accounting, I think it does a pretty good job of giving an overall picture of dominance within a given year. Notably not on the list above are Agassi, Edberg, Becker, Courier, and, well, everyone else. And most importantly: it gives us a way to assess just how good Sinner has been this year.
TLDR: There is every reason to think that both Alcaraz and Sinner are in the process of ATG careers. The question is
how great. And yes, it is debatable whether or not they remain this dominance, but given their age and trajectories of improvement (Alcaraz more gradual but younger, Sinner more steep but older), I suspect we haven't even seen their best yet. Or rather, if I were to guess, this will likely be among the best seasons Sinner ever has, but Alcaraz still has great heights that we haven't even seen yet.