At Marvel, they have worked admirably to create a franchise out of what they were left with in terms of properties, but that can't continue forever. Just look at the statistics, the merchandising sales for Spider-Man were some 3 or 4 times that of every single MCU character. That is the difference between A-list and everything beneath it, even through Iron Man and Cap were B-list. As Marvel digs themselves deeper and deeper into their stock of properties, their situation will become just that: a hole they cannot climb out of. The increasing lack of iconography to their characters means they will resort to the same tired formulas of plot and characterization to hope for the same success for new characters as old ones, something audiences will get bored of. Sure Guardians shows any character can be a success with the right marketing, but those characters were moreso tropes to fit in a team dynamic than ones with depth. There's no simple stereotype to be applied to a character like Black Panther or Captain Marvel, they actually have to try. Many people already are getting tired of the commonalities of Marvel films like the poor villains, macguffin plots, poor (literally interchangeable; see: Wright) directing, forced characterizations, etc. so there's no way they'd accept that keep accepting that in the future and Marvel hasn't shown they can do anything else. This explains their desperation to get even guys like the Fantastic Four back since at least people recognize them. They know their efforts can only go so far with characters audiences don't even have in their peripheral vision.

Meanwhile at DC, they own every single one of their properties. Sure there have been some real shitshows like Green Lantern or Jonah Hex, but now with this new cinematic universe they are more focused creatively and financially than they ever have been. They now have a real vision for these films, and moreover Oscar-winning talent at WB to back it up. Hell, WB basically invented the comic book movie genre and are primed to retake it. Speaking of iconography, the Dark Knight trilogy and Man of Steel show that these immortal characters in the DC lineup are ones that audiences actually want to see done well. Those Batman films made a billions (also Batman merchandising sales are 2 or 3 times higher than the whole MCU) and Man of Steel's divisive reception shows to me more than anything the respect people have for these characters and the desire they have to see them on screen and done well. With the slate of DC movies, people will go see them because they want to see these characters, unlike with Marvel where they go just because it's a Marvel movie and they hope for the best. The latter's good will/humor from Guardians can only old for so long (just imagine an Iron Man 2 level film but with a new property, everyone would despise it and that character would never be seen again). Snyder and company have definitly learned form Man of Steel the boundaries they have in front of them, and they will only grow and improve moving forward (proof: David Goyer is gone). I have zero doubt in my mind that Batman v Superman will outgross Avengers and Age of Ultron.