I agree on the perception being mostly anecdotal. The number of true Disney nuts is pretty small in comparison to the number of people who actually went to the Avenger movies. While there is a subsection of pixie dusters who suddenly became Marvel fans there are probably also some teens and young adults who are turned off by Disney's involvement since Disney is for kids.
Probably a large portion of people who saw Marvel movies aren't even aware that Disney owns the rights. Many who weren't into comic books don't understand why Superman and Batman don't join Captain America and Spider-Man. This is just anecdotal evidence as well, but to your point on the costumes, between my 2 kids they have been all 4 of the major Avengers for Halloween. They have no idea Disney owns the rights to the movies especially since the characters aren't at WDW. They also went as Batman and Robin one year. My oldest son does know which characters are Marvel vs DC comics. Not sure how, must be something the kids in school talk about.
I think both sides are correct in this debate. There are definitely some Disney fanboys who only have interest in Marvel and Star Wars since Disney bought the products. On the other side those fanboys are not why the Marvel movies were successful. They make up an insignificant portion of the total viewing public. The Marvel movies made hundreds of millions at the box office. That's not just blind Disney loyalty.