Memes aside, Hamilton didn't go to Ferrari because he believes he will win. He saw what Mercedes was doing and he realized that he can't get a win there, no titles, so he might as well go race for a few years as the most known brand all around, Ferrari is THE racing team, in all over the world at all kinds of races.
And now, he is just having fun, doesn't even care that much, care enough that he wants to give his all, but he is not expecting a title. He knows his title times are gone, so he just focuses on being happier and just be a Ferrari guy. So there is no way he would go back to Mercedes, there isn't anything that makes sense for him to go there. With that being the case, I am sure that we are going to see him stay there, and Max will be at Mercedes instead.
Hamilton has now closed an era. Because so far, after championships, countless poles, victories and records, he has managed to become one of the best in F1. He also recognized Mercedes' decline early on. The technical difficulties, some strategic mistakes within the team showed Hamilton that it was very difficult for Mercedes to return to the top anytime soon.
I think Hamilton is now looking to add a story to his career. Moving to Ferrari is a career symbol because almost everyone who has become a legend in F1 history has crossed paths with Ferrari in one way or another. Max Verstappen moving to Mercedes is a possibility. If the internal turmoil at Red Bull grows, it could be attractive for Max Verstappen to create a new legend at a big brand like Mercedes.