You have to remember, if you don't read comic books, nearly EVERY hero is from the "back-catalog." Beyond the Hulk (who was in a TV series and a couple of movies), and a passing familiarity that Captain America existed at one time, the remaining characters in the Marvel Cinematic Universe are virtually complete unknowns to the general public. Did a bunch of the other ones have active comic books going on when the movies were planned and were huge "hits" in every comic book store in the land? Maybe; I have no idea, and neither does most of the movie-going public. (It's telling that comic books aren't sold at general-interest stores anymore; the only place to buy them is specialty comic and hobby shops.)
And if Marvel can make Captain America, an obvious bit of cheesy WWII gung-ho patriotism, work for a modern audience, I don't see how you can say that Wonder Womam, Flash, and The Green Lantern "haven't aged well." They don't NEED a current "following" to be successful in a movie... the number of people that are the audience for comic-book movies is SO large compared to the number of comic book buyers, that it doesn't really matter how popular the hero is in current comic books; the people that buy comic books are only a tiny portion of the target audience.
The one any ONLY reason that the Marvel Cinematic Universe has worked out so well (without using the most recognizable properties in the stable) is due to Kevin Feige's unifying long-term vision. At the time Ironman rolled out the door, he had at least a hint of a plan ready to go, and quickly pulled it into a quality long-term plan tying everything together. But if every movie is planned out by a different producer, then it becomes Just A Bunch of Comic Book Movies. Would The Avengers have been nearly as successful if it merely happened to contain heroes from previous movie franchises? Heck no.
But DC clearly doesn't understand this, as the movie they are counting on to revive their fortunes, "Batman vs. Superman", contains two heroes who have never met or discussed each other, at all, in their respective movie franchises. Mashing them together is going to look like just that, a mash-up. When Avengers rolled out the door, the ONLY new characters were Hawkeye (yes, I know he had a cameo in Thor), and Smulders. The rest had been introduced quite clearly in previous movies, and were also clearly part of an over-arching theme. (Introduced first in the little credit snippets, and then rolling out to larger roles in the movies.)