Alekhine toyed with Capa for a decade, not giving him a rematch as payback for Capa's having done something similar to him previously. Fischer won the title and quit. It is not that Magnus is no good, it's that he isn't fully living up to his name (Magnus means great), which disappoints slightly (irrespective of one's chess rating, of course).
(Chess has become more and more "political" over time. Fischer had no respect for Karpov or Kasparov because of the things they had to do to be on top. Fabio and Magnus play the needful game off the board but at least Fabio makes some reference to "the powers that be." It is only natural for Maggie to want to remain champion of the world, it's only human. We would honor him, though, if he continued to show the fighting spirit he did in the past instead of acting like an old man already at what, 26? Unless you count Morphy's losing it, I think that's another world record.... Advance apologies for speaking so brashly here, I didn't realize I'd become such a hero-worshipper. I guess it's only natural now that /It's a hard world to get a break in /All the good things have been taken.)
Caruana had to prove that he can beat a declined and in bad form Carlsen and he failed to do so.