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.