I think the idea of no more "stand alone" IM movies with Downy Jr is a smart move, if not box office-wise, at least creative-wise.
Smart of Downy to walk away while the series is still hot...make a few more appearance as the character in ensemble pieces (Avengers) and leave his contribution at three movies (well, 4 if you include Avengers) that have had the character of Tony Stark go through a complete story arc.
He's in good company for the walk-away...Damon leaving the "Bourne" movies after three, Bale hanging up the cape after three, and both of those men (along with Downy Jr) have imbued these characters with their strong personalities and screen presences, so while it's hard to imagine anyone else in this film era effectively playing these parts, at least they can walk away with some solid, well-appreciated work behind them that will sand the test of time.
Can someone else play Stark down the line? Most likely. Will it be the same Stark we're experiencing with these movies now? Heck no...but hopefully, if the character is re-visited, it will be a totally new take on it...more like what it appears Synder may be doing with Batman opposed to what the already-tired and too soon re-do the recent Spiderman was (sorry...tried watching it on cable recently...couldn't do it...it missed the mark so badly, it's unwatchable).
I've really enjoyed Downy's take on Tony Stark. We've got a couple more movies to see where he goes with the character...but three movies with him as the center of the story is enough....
