For some concrete numbers, there are only four players over 50 years of age in the top 100 at the moment by live ratings[0]. They are ranked #13 (age 56), #89 (age 53), #95 (age 54), and #97 (age 57). In their primes these players were ranked #1, #10, #4, and #3 respectively.
Isn't he playing Chess960 because he started finding standard chess boring? And wasn't that why Fischer worked on it in the first place? Experts might get bored of it by the time they're 50.
The reason the top pros like chess960 is because they don’t need to spend hundreds of hours of opening preparation, they can just sit down and play.
Caruana (the guy who lost to Magnus), mused in a podcast that chess960 feels strange as a competitor because he doesn’t really prepare (because there are far too many openings to study) and said it feels like he’s getting paid for much less work.
There are 960 possible starting positions and the chosen one is known at the start of the tournament where players are given 15m to prepare. I have observed that GMs aren't surprised when they see the board. They usually go "ah it's this one with the opposite bishops" or something similar.
When a chess player means "no prep" it probably still means more prep than any normal person would consider reasonable, because what would require you to sit down and take notes, move pieces and memorize, they can just do in their head getting coffee by now. So yeah they recognize almost all the patterns, it's just harder justify spending 1 month on an opening you won't even be able to use, but they still know how to play certain patterns.
Oh, totally, I just wanted to highlight what beasts these players are and how wonderous it is to see them recognize so many starting positions that they already started showing familiarity despite how new the tournament format is.
[0]: https://2700chess.com/?per-page=100