Some good questions.
It's hard for me to imagine Magnus slipping down the ratings chart.
I expect he'll step away from competitive play while still on top, much like he did with the World Championship. He's better aware of his abilities than anyone - and he'd be able to tell when the next generation is ready to dethrone him.
For now, though, I suspect he still has several years more to play as the best player in the world. His interviews during the Candidates seemed rather "ho hum" - as if he wasn't impressed at all by the level of play that he was seeing.
I imagine it gets a bit boring being on such a high mountaintop, all by himself. Top players like Fabiano and Hikaru were wracking their brains to find the best moves in the final round, and Magnus was watching and shrugging - seeing everything that they missed ...
It can be difficult for us to see it, because all the top players are so highly skilled. But Magnus still towers above them.
As we all know. Magnus Carlsen withdrew from defending his WC title, and has effectively "retired undefeated," in the manner of old boxers like Rocky Marciano.
Officially he held the title for 10 years, not a record by any means. He actually defended it for 8 years: after winning the crown in 2013 against Anand, his last (successful) defense was 8 years later in 2021 against Nepomniachtchi.
So, what to make of this? Should Kasparov have done the same thing - quietly retire after almost 15 years of holding the crown, and not go down in a flame of Ruy Lopez Berlins against Kramnik, in one of of the most mind-numbingly boring WC matches ever?
More important - where is the Carlsen legacy going to land after all this? Did he jump too soon? He did defend his title four times, which is a lot - although in a tight 7-year span. Most "Best of" lists now have him in the Top 3, which isn't an easy level to crack. Think he'll stay there, as the years rumble on? Or will he slowly slip down the Top 10 like others have done (or like Kramnik and Spassky, just fall off completely).