It applies to both short and long term, I think perhaps you are taking that phrase to mean something a bit different than it's meant to be. It boils down to this - if there's no challenger (or challengers) for hero who stands to be the opposite of who the hero is the story doesn't have as much significance because there's ultimately not as much at stake, whether a short story or a century old mythology. He's just the hero if there's not a strong supporting cast of characters, both good and bad, around him to help flesh him out. I've heard with regards to The Dark Knight from some quarters that it's not a Batman film because he's not front and centre. The counter argument to that is that Batman is more than just Bruce Wayne in costume, it's an entire mythology.
You could very well make a movie about any hero, have him/her be front and centre the entire time and it could turn out perfectly acceptable, but at the end of day the from a story telling perspective that type of heroes journey lacks depth. If your argument is that because the movies haven't been awful because of the focus on the heroes then I can't really counter that, but all that really means is they are characters of an acceptable standard and play to easy beats, and in some cases are probably elevated more by the actor playing him/her. The truth it we really don't know how good most of these characters are or what their potential is because most haven't had to face their opposite number, with the exception of Thor.
But then again maybe people are simply ok with that type of depth these days, I don't know, maybe peoples tastes have evolved thanks to the type of movies in general being made in recent times, safe and sanitised experiences that are generally inoffensive and do just enough to win people over. Mad Max Fury Road was one of the most insane movies of the last 15 years with a memorable bad guy yet most people avoided it because it was so different. It is what it is I guess.