Why focus on how Devin is making the argument rather than the substance of the argument?
Devin is saying that superheroes from comics have fallen short of becoming a “new American mythology” as he had hoped because their popularity has led to their commodification, which requires a kind of reduction to the lowest common denominator. You can argue that comics have always been commodified, but we just forget the junk and remember the masterpieces. You can argue that comic movies may include dumb stuff but still are contributing to a “new American mythology.”
But just declaring it gatekeeping is not moving the conversation forward. If you want to defend the movies, do so. Tell Devin why he is wrong, not why he is wrong for saying it that way.
