Magnus played 5 world championships, with 3 against players of his generation. In those 3,he only managed a plus score once - against Nepo who was more than holding his own then lost one tough game and went on his notorious monkey tilt. The other two were drawn in classical.
Magnus is, by a landslide, the best tournament player (probably ever) but the world championship for classical is very different than a tournament, and his results there have not been anywhere near the level of his tournament performances.
And Magnus has also stated that he believes he has peaked. Basically - he was going to imminently lose, and I think he wanted to go out undefeated. Notably the one player he was willing to play, Alireza, was the only viable contender who he would expect to have been an overwhelming favourite against.
Also in terms of legacy, the max number of world championship victories is 6. He stopped at 5.
It's also worth noting that is he is the reigning Rapid World Champion, the reigning Blitz World Champion, and the reigning Chess World Cup Champion.
He chose not to defend his Classical world title, and has been quite clear about the reasons.