None of Marvel's films have depended on star power and I don't know why you think they'd start now.
Well, I disagree, at least when it comes to the Iron Man films, however you're right about their other films, but they've still always took people with comedic background and with charisma who makes great leads for their roles. Asa doesn't fit into this either and Spidey is the one who needs a charming funny lead more than ANY of them.