Magnus has specifically said that he no longer wants to play the world championship. He says that this does not mean retirement, on the contrary, he wants to continue playing tournaments in order to reach 2900 Elo points. I'm kind of skeptical let's see what happens.
Sure, 27 years is a huge period of time. As Mijail Tal used to say: "I'm not worry about how long I can hold the title of world champion because this title is provisional while the title of ex-world champion is eternal."