There's no way they'd ever kill off Tony Stark. This is the Marvel Cinematic Universe -- every single one of the movies thus far has been the epitome of "playing it safe." When Downey, Jr., decides that he's done with the role, it'll be re-cast; Kevin Feige has said as much.
But Marvel also kills of characters and then brings them back. Sure we are all use to the schtick (though I really thought Cap was dead after Civil War) but movie goer aren't so sophisticated.
Exactly, which is why none of the heroes will ever be killed. These films play it safe to an almost insulting degree.
The Marvel movies, especially The Avengers
, are sandbox movies. For example, playing with action figures in the sandbox, you'd have Thor fight Iron Man for no reason other than you thought it'd be cool. There is no way in hell that Disney would ever give the green light to knocking off Tony Stark, especially given that the character's movies make the most money of any individual character.