I think that to most audiences RDJ IS Tony Stark moreso than Tony Stark is Iron Man. Especially as most don't know Iron Man from the comics and know him only as RDJ. Anyway, I don't see why there would necessarily need to be an Iron Man or other armored Avenger at all times. There are many other characters they could use instead who have just as much potential to become popular. Especially since people are more willing to give Marvel a chance than they were before the MCU got big. I mean, there are people now who never read a comic that have Bucky Barnes and Star-Lord as their favorite characters now. Also, recasting also means that you won't get the chance to bring the character back in a smaller role with the original actor in the role several years later. Like, I'd rather focus on different characters after Infinity War, but maybe have RDJ return when he's in his 60s in a role similar to that of Michael Douglas as Hank Pym, as CEO of Stark Industries, or an important member of SHIELD, or the United States Secretary of Defense or whatever. You can't do that if you recast. This is a unique chance to see what a Marvel Universe would be like without some ridiculous sliding timescale, where not everything always returns to status quo.