The ultimate selling point of The Expendables franchise has largely been similar to The Avengers. Certainly they could not be more different in overall tone, and there is no continuity experiment at work. But at their most basic, they are both ensemble films offering the previously unheard of opportunity to see all your favorite childhood heroes teaming up in the same movie. For Avengers it was the comic book characters they grew up reading. For Expendables it's the action stars of the movies of their teens. But there is a reason why Avengers worked and the Expendables films absolutely do not. Comic book superheroes, whether because of their adaptability, their universal themes of teamwork and altruism, or their fantastical detachment from reality that keeps them from being anchored to one setting or time period, have a timeless quality to them. The Avengers was about characters that have been enjoyed by generation after generation of readers. It was a movie that touched that small shred of childhood innocence every jaded adult still has and taught us to cheer and have fun again. The Expendables, by contrast, is anchored unshakably to one long dead era in action filmmaking.
