Lets assume just for the sake of argument that Magnus has insider information from chess.com making him 98% certain that Niemann is cheating.

Why would he hand him a game that's going to be watched worldwide, where Magnus has nothing to win. Since if he wins we really still don't know anything one way or the other. But he also has everything to lose. If Hans is cheating and manages to pull off something again, then Magnus is cripeling his own reputation.

Magnus seems to be doing the right thing here, which is voicing his concerns, refusing to play him, and asking Niemann for permission to speak on the matter fully. Niemann is doing what you'd expect of a cheater, which is to stay quiet, dismiss the discussions, having difficulty explaining his plays, and pretty much just holding back from letting chess.com or Magnus divulge what information they have from the inside of his bans.

I mean, Magnus is so much more better player than Hans that even if Hans didn't cheat he would probably be worse in rematch. But in all sports sometimes underdogs win. In couple yt videos i watched it was said that Magnus played bad game and that Hans already gained an advantage in opening. Hans said that he prepared opening play but if he is indeed a cheater he maybe used engine just for opening. We'll probably never get an answer if there was cheating or not

But if he cheated a lot more (as Magnus seems to allege) then he'll also lose to others he used to beat easily.

EDIT: I think a big issue with chess is this "perfection" mentality. Magnus cannot make a single mistake or lose a single game without it being a big show. Hans beating Magnus this one time, even though he was playing against black, should basically mean nothing - Magnus can probably beat Hans 10 times over. If Hans was beating Magnus 10 times it would be different - but also if he was on a streak to Magnus 10 times he would be playing with a lot more scrutiny.