This seems accurate to me as a backward looking assessment - essentially, how good players have been in their careers. Partey has played at a very high level in spurts but never had the consistency and ability to stay healthy to be ranked in a higher tier. I don't see him getting healthier at this point in his career and he obviously has another issue not to be talked about in this thread that could impact his availability as well.
But I don't think I would bet on Casemiro or Fabinho to be better than him going forward (assuming he stays unsuspended for the moment). As the PL became a press heavy, higher tempo league in the last 7-8 years, it has been a complete graveyard for top level central midfielders past 30 - Cazorla was done at 31, Schweinsteiger done at 31, Dembele done at 31, Fabregas done at 31, Matic done as a top level player at 31, Kante seems done as a top level player at 31, same with Jorginho, Thiago is still tidy at 31 but can't really keep up physically, Henderson looks totally cooked having just hit 32, etc. The game is just so physically demanding now for CMs, guys just don't have the legs to keep up past 30 and often their bodies break down as they try. The only real exception during this period was Fernandinho and, likely not coincidentally, he played for a side that other teams gave up trying to press and spends 80% of the game camped in one half of the field rather than going end-to-end all match.
Casemiro is 31 in four months. Until proven otherwise, the safe bet is that he goes on the trajectory of these other CMs and struggles mightily with the physical strains of the PL, never playing at a consistent elite level again. Fabinho will only turn 29 later this month, but he has played an unusually massive number of first team minutes in his career (more than older players like Casemiro, Thiago, or Kante for example) so he seems a likely candidate for an early decline.
The best bets to play at a top level in the PL over the next couple years are Rodri, Guimaraes, and Rice. Hojbjerg is not close to them IMO.