Mic-Nificent
I didn't eat nobody
Even though the character had been around for decades, there had never been a live action Ironman. And most people who've seen the movies never read the comics, so Tony Stark was as fresh as Jack Sparrow to them.
That's true, but what has RDJ done with the character that another actor couldn't (besides be the first to play Tony Stark)?
There's nothing about Downy's performance that makes him irreplaceable. He didn't bring anything to the character that wasn't already there. Just get another good charismatic actor to fill the role.
If the Iron Man franchise is in any danger it's not because of RDJ possibly stepping away, it's because they've completely ran out of Villains for him to fight. Iron Man has the weakest Rogue's gallery out of the major Marvel characters.