Re: Downey's last?
I think RDJ gets killed off in Avengers 2 or 3. The excitement of IM solo movies lost a little luster after Avengers. We'll see if this movie changes that (not in terms of box office but in desire for more stories). I think Marvel should retire the character because clearly anything that follows in the near future this is going to get lambasted by the public.
You reboot Iron Man after all characters and back in house, or at least Spiderman and the Fantastic Four, which should be another 10-15 years tops. When I say reboot, don't start the MCU from scratch. Just use select stories involving characters and any point in their history and tell that story. That way you can cast a 20 year old Stark for the teeney bopper version one year, and a 50 year old Stark for a more adult oriented version, or veteran superhero cross over flick.