I think you're mostly right in that ALL Slams were dominated by three players (with a bit of others sprinkled in). It is very unlikely that Sinner, Alcaraz, (maybe Fonzie etc) will be as dominant.
But...I think Roland Garros is a bit different because of a combination of factors:
- It was dominated mostly by one guy who won 14 of 18 of them from 2005-22
- In the preceding era (1989-2004) it was more prone to "one Slam wonders," with no player winning it more than three times (Kuerten) - compared to greater "ATG dominance" at the other three Slams (see below).
Let's look at 1989-2004:
AO (9 winners): Agassi 4; Lendl. Becker, Courier, Sampras 2 each; Korda, Kafelnikov, Johansson, Federer 1 each
RG (12 winners): Kuerten 3; Bruguera, Courier 2 each; Chang, Gomez, Muster, Kafelnikov, Moya, Agassi, Costa, Ferrero, Gaudio 1 each
WIM (9 winners): Sampras 7; Federer 2; Becker, Edberg, Agassi, Stich, Ivanisevic, Krajicek, Hewitt 1 each
USO (9 winners): Sampras 5; Edberg, Agassi, Rafter 2 each; Becker, Federer, Roddick, Hewitt, Safin 1 each
So the other three had 9 unique winners, the French Open 12. But more importantly, only one of those RGs were won by an ATG (Agassi), while 11 AOs, 12 Wims, and 11 USOs were won by ATGs.
Meaning, at least in that era, RG was far more prone to have lesser players win it than the other three Slams. I think a key difference, obviously, is there is less of a gap between clay and other surfaces these days ,which also meant that there were more clay specialists back then (and before...think guys like Clerc). I'm not sure we can equate someone like Cerundolo with Albert Costa...maybe Cerundolo would have been more of an RG threat in the 90s and Costa less of one today. I mean, Casper Ruud probably would have been Thomas Muster and won a bunch of clay Masters and a Slam or two.
Probably what we'll see is similar patterns as AO/WIM/USO in all four Slams today...mostly won by greats (Alcaraz, Sinner, and whoever joins them), but with a handful of lesser players winning Slams - and more than the 2004-23 era. No Rafa, of course.