Re: Downey's last?
The more I think about it, the more I believe after RDJ we're gonna need someone to be the sacrificial lamb...so to speak. I mean, think about it. Both the Bond movies and the Superman* movies(and even the Batman movies to a lesser extent) had this happen. Lazenby, Routh(and even Kilmer & Clooney) came in as filler for one movie so the audience could say "Bah, he's nowhere near as good as 'so & so'" and then they dump the actor for someone else and by then the transition isn't so hard to take. It's uncertain whether Andrew Garfield will suffer this fate in the upcoming ASM movie. Unlike Connery/Bond, Reeves/Superman or RDJ/Iron Man; I think Maguire's Spider-Man was more in line with Keaton's Batman in the sense that both did an obviously acceptable job but never completely nailed the role like the first 3 I mentioned did. So we may be getting an actor as Stark for a single film that may not do so hot and then get another recast that is accepted better.
*Of course this still has yet to be fully realized here but I feel confident that Henry Cavill will own the role much more than Routh ever did. It's not exactly a hard act to follow.
Current comic book TV show grades(ones that I watch, at least): Agents of Shield-[season 1: B-][season 2: B+]\The Flash-[season 1: B+/A-]\Arrow-[season 1: B+][season 2: B][season 3: C]\Agent Carter-[season 1: A-]\Daredevil-[season 1: A+]\iZombie-[season 1: A-]