The X-Men aren't about Villains. Get it? The X-Men are superheroes. Not the bad Mutants club.
And I don't care if Venom and Joker were successful at the boX office... any comicbook character could achieve successful box office results, depending on either marketing or quality or both or for other reasons. As for Marvel films, I just prefer having the spotlight on the heroes. So give me the 3rd Fantastic Four reboot, Nova, rebooted Daredevil film, rebooted Iron Fist film, rebooted Inhumans, the rebooted X-Men, Black Knight solo film over a film/tv show focusing on Dr. Doom, Loki, Magneto or whoever Marvel villains out there.
And as for Thanos, he didn't headline a solo film. Infinity War and Endgame are Avengers movies, not Thanos movies.