They watched friends, allies, brothers, sisters, and lovers die in a fight that none of them chose. Why would they forsake those deaths by pretending to be the very things that sent them there?
You are also assuming that none of the villains have *anyone* left behind on Earth that their sacrifices are saving? Given that not all Villains are in it for world domination. Sandman, Rhino, Juggernaut, alot of the 'henchmen' types, if you suddenly succeeded in this *massive* mission and saved the earth, wouldn't that be enough to rethink your motivations? Coming home to the family, and friends now cheering you, whom you haven't spoken to in years because of the moral choices, or hardships of villainy you've endured? Why wouldn't you be at least tempted to turn it in. The public loves you, now, I am sure the government would be willing to overlook your previous record in light of your accomplishments.
The heroes made the choice, not the world itself, why not use your greatest tool, this new found popularity to stick it to the old guard heroes, and remain in that feel good public eye by becoming the heroes for the people you just risked your life to save.
Am I simplifying it? As much as any campaign is simplified, because I feel it gives the characters more conflict, and more depth, and more struggle. Us vs. the World is boring, Us vs. Our Rival Heroes on the Public Stage, Us vs. Newly formed Super Criminals, and Us vs. Ourselves are all much more compelling in my minds eye.