Well he's going to be replaced eventually however I wouldn't want it to be in THIS cinematic universe

As I was saying in another article, replacing actors is fine IF it makes sense story wise. Take Captain America for example it makes sense if he is replaced in this universe since Bucky took over in the comic and just recently Falcon however if you replaced Tony Stark or even Thor with new actors with no storyline which explains the change then it's going to look weird and off putting. Especially when Disney might be able to convince Scarlett Johansson to stay on as Black Widow and Don Cheadle as Warmachine yet they would be staring alongside a new actor for Ton Stark...mixing old and new together. In the end it just wouldn't work as mixing old and new actors will not give us the same chemistry on screen they've already built up alongside the old actor. Look how bad it was when Don Cheadle took over as Warmachine from Terrence Howard, the chemistry had to be built up again and that was just ONE FILM.

The best thing I can think of is the studio just pay the actors to reprise the roles in new films once every so often. We don't need film after film from them, just when they need the character to further the current or next Phase of films. So we could have Iron Man 4 at the very start of Phase 4 and not see Iron Man 5 until the very end of Phase 5.

If they ever touch on the Civil War story line than Iron Man will play a major factor, but I think its time to move on and focus on other characters.