I hope that even if Downey doesn't do anymore solo films after IM3, he sticks around for two more Avengers movies. But I can understand if he decides to hang up the armor sooner than later, given his age and perhaps not wanting to return to the same well too many times.
I think I'd prefer recasting over rebooting, too, but I don't know who could be a viable successor to Downey as Iron Man. Maybe Colin Farrell? (Joking.