Pretty sure RDJ said that he would do Iron Man for as long as he enjoys playing the character.
Which pretty sure he does. So as long as they continue to keep the stories good and production value good, I dont' see him going anywhere.
As for this movie outselling Avengers. i doubt it, btu who cares, it will sell probably 1 bn or around that. I'm just loving the whole comic movies these days. they are so frequent and popular and "Good"
Remember back when we got maybe a comic book movie like once very couple years at best. Now we get like 3 a year. It's awesome.
edit: and anyone who hasn't seen Sherlock (bbc) shoudl go and watch it. It's on netflix. Amazing show. Best sherlock adaptation i've seen. better than the movies and better thant he elementary show. oh and bilbo baggins is watson
