Those were origin stories. Start with a team movie and there's a LOT you have to explain to people on powers, abilities, personalities, etc. Or just assume the audience knows this and roll with it.
was pretty much a team movie and it succeeded. Same with Monsters vs. Aliens
-- there were lots of new characters with their own specific origins to introduce, and the movie got it out of the way in a brief, pithy montage. And I've listed a number of non-superhero team movies that succeeded as well.
I don't think origin stories are really all that necessary for every hero. All you have to do is establish a world where superheroes are a fact of life. Even an audience that isn't familiar with the details of every superhero's past is going to know the basic archetypes -- badass-normal vigilante, tech-enhanced genius, superpowered alien, mythological magic-user from a lost civilization, things like that. So those archetypes could be established as easily as Monsters vs. Aliens
established its movie-monster archetypes. And it's not as if DC's heroes tend to have backstories as complicated as Marvel's anyway. It wouldn't be that hard to establish the basics of each character's powers and background in a matter of moments, and after that it's simply a matter of building characterizations the same way any ensemble-cast film does.
I can understand trying to explain a film's failure after it's already failed. I cannot understand why you're going to such great lengths to make excuses for the presumed failure of a film that hasn't even been made yet - and that, for all any of us knows, might not even fail at all. Why not just wait and see what actually happens instead of putting so much effort into manufacturing a pessimistic future that might not actually come to pass?