No
If Magnus quit chess for 20 years and then decided to come back, what would his rating be?
Yes.
Fischer came back in 1992 after 20 years of inactivity and was still grandmaster strength 2650.
http://www.chessmetrics.com/cm/CM2/PlayerProfile.asp?Params=199510SSSSS3S038178000000111000000000000010100


I have come back after a 25-year hiatus, my chess skills suffered, but there is muscle memory. All the forgotten patterns, openings and tactics are beginning to emerge again. I cannot play at my old club/tournament level of 1600-1800, but I can still play a reasonable game, and I'm improving almost daily.
A super-GM would lose some ability but, his muscle memory would be impressive. I would expect 2500+ Elo within a short period and advance slowly after that as the skills return
Karpov
Korchnoi
Is it just Russian Grand Masters, whose names start with "K", that can remain so good as they age?

Would a 50-year-old Magnus Carlsen who hadn't even looked at a chess board for 20 years still be GM strength?
In the 2500s I imagine.
Oh ok, maybe we're assuming he actually prepares and works on chess for a year before returning... then I guess at least 2600s.

Would a 50-year-old Magnus Carlsen who hadn't even looked at a chess board for 20 years still be GM strength?
Yes. Easily.

The better the player, the more things they do automatically, such as certain moves they wont consider. It's just how their brain is wired, it's not even a conscious act anymore.
So even with extreme rust (like not playing or even thinking about chess for 20 years) it's hard to fall more than 400 points.

If chess were about how much you know, then we'd never have 20 year old world champions.
Chess is performance as much as knowledge. It doesn't matter how much he'd remember.

If he had amnesia and couldn't remember his name that would matter 😂
But sure, I guess I overstated it. My point was even if he remembered everything, he wouldn't be the same rating.

I already said, chess isn't about who knows more. Chess, at least live games, is a performance.
It's sort of like thinking the person who wins gold for Olympic gymnastics knows the most about flips. That's just silly. Their 60 year old coach probably knows more about flipping in the air, but their 60 year old body can't do it anymore. It's the same for chess (but obviously not that extreme).
The average age of the top 10 in the world was just under 30 years old last time I checked.
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I mean, obviously knowledge helps, but by itself that can't wins chess games.
Would a 50-year-old Magnus Carlsen who hadn't even looked at a chess board for 20 years still be GM strength?