Re: Downey's last?
I don't think Tony Stark will die, he will probably leave the universe for a few years before a new actor takes over.
He hasn't died in the comics and has lasted for 50years now.
The run from IronMan1 to Avengers could have just spanned 3years, so whos to say how long the universe can run on the characters without a reboot.
Just swap and change the members of Avengers like in the comics.