I think he can keep playing this role for a long time as long as he stays in shape. With the miracles in visuals and CG, it won't be hard to make him look younger or up to par on screen.
Even Roger Moore and Sean Connery played Bond well into their 50's. And Connery started the role as a young man who had competed in bodybuilding competitions. Though Moore started much, much older. He just can't get like Shatner and get tubby when he gets old
But yeah I really can't see anyone else playing the role besides Downey at this point. Though realistically, I'm sure someone could. I just don't want to see someone else do it
. However, I see no reason he can't do it well into his 50's really.