Re: Downey's last?
Downey's replacement should be as young as Chris Evans to fill in many movies and would look as smart as Downey so people won't miss RDJ. Iron Man doesn't need to be an American so I seriously think the next actor will be outsourced. Personally I find Farrell acting is too dark as Downey substitute.
I don't know if they will throw the Teen Tony curve to compensate a younger Tony Stark actor. Either that or bring Black Panther/Ant Man as the Avengers scientist-technologist.