Plus people are realizing that, hey this Iron Man character is actually pretty awesome. Robert Downey Jr's incredible charisma is also really helping things in that respect.
I'd bet that it's the opposite of what you're saying. Robert Downey first, THEN the "Iron Man character is pretty awesome."
Because Tony sure has changed from the comics and previous cartoons. It's all Robert Downey Jr and they're screwed when he wants to leave.
Luckily, he seems to still be having fun.