Why did Magnus retire anyway? I don't really follow chess but watch a chess streamer (ChessNetwork) on Twitch from time to time.
After more than 10 years of first trying to gain the title and then maintain it after he won it, he just lost motivation. Plus, it's a lot of physical and mental strain to prepare for and then play a WC match, and he did it 5 times already. Does he really need to prove himself anymore against the players of his generation? But yeah, I guess the main reason is, it stopped being fun for him.
Of course he didn't retire from chess, he just doesn't participate in the WC cycle anymore, but for example he took part in the speed chess world championship last december and won both the rapid and blitz titles.