Re: Downey's last?
I'm really, really averse to seeing another actor play Tony Stark. I sincerely hope that RDJ will make a new contract with Marvel when his current one is done and come back for at least two more solo Iron Man movies, and two more Avengers. He's stated his love for the character, and that he'd be happy bouncing back and forth between Tony Stark and Sherlock Holmes for the next ten years.
That said, Tony Stark is an enduring character that can survive a recast imo, if that recast is done well. If the MCU is to endure, recasts should be considered for all of the characters sooner or later. Bringing in new characters is risky, riskier than recasts. At least that's my take on it.