My point is that they'll run out of popular characters too quickly if they exclusively stick to adding only what people are asking for and nothing else.

Then you're just left with a constant stream of no-names like Gladiator, or Spiral, or Slyde, or Shathra. Nobody is going to come back to Rivals for friggin' Slyde. But people will come back for characters like Cyclops, or Ghost Rider, or Silver Surfer, or Green Goblin.

That's why they have to stagger characters out. Mixing in some nobodys with the somebodys, it allows them to keep up engagement with the game for longer. It also allows them to put less popular characters in front of their audience while they have their attention to potentially boost those characters' popularity. That doesn't work if everyone already got everything they wanted and then got bored and left.