OP I'll tell you what it is
it's comic book readers
they're predominantly older, male, sometime in their probably early 50s now, with a family and a lot of free capital (high earners)
there's fewer and fewer of them every year, because the older ones are dying off
what you get as a subset of these is very vocal fans who fit the basic profile but maybe don't have kids, or still read comics to escape their families even though they don't have a lot of money
and this subset are the superfans: the ones who know every storyline from the last 10 years, even the ones they hated
they read it all, and they get angry about it, and to be honest, that vocal anger can sometimes drive the publisher's offerings if the publisher mistakes them for the majority viewpoint because of how loud they get
so it is with movies: they've followed the comics for years and they feel that publishers "owe" them (in fact, the transaction "money for comics" has been satisfactorily completed every time they've bought anything) and by extension production companies working on movies about those selfsame comic book characters "owe" them (which is ludicrous)
and when you get a movie that's not aimed at this subset, or which isn't even aimed in their general direction (such as the first two Thor movies), they complain
a lot
and they will never stop complaining, because they feel that they somehow "own" the characters, they're emotionally invested in them and their worlds; broken little men playing with dolls
ignore them, most people are normal, even among comic book readers