Right. It's not going to happen this 'generation.' If it ever does occur, it will be far enough down the road, that, much like a Keaton/Reeve teamup, a Downey/Jackman teamup (two pairs who equally exemplified their respective characters, gaining iconic, almost exclusionary, status in each role), will probably never happen. Reboot(s) will have undoubtedly occurred, and we will either have a new Marvel film universe that doesn't have to eschew the word 'mutant' in favor of 'inhuman,' or in true comic book movie history fashion, the alteration from 'mutant' to 'inhuman' could filter into the comics and become the new canon, or at least the canon of a new universe, much like the Ultimate universe.
Another possibility is that the MCCU will continue, and eventually move past the original characters to tell other stories, without bothering to reboot the old ones. Characters like Iron Man could be replaced by characters such as Dr Strange. New headliners could step in to take the spotlight. Iron Man himself was once a second stringer before achieving the level of popularity he has now. Who's to say he needs to be recast/rebooted once Downey leaves the picture, if other second-stringers could potentially attain the same level of success? While I see it as unlikely, it is certainly interesting to entertain the prospect that the Marvel film universe could 'retire the jersey,' so to speak, once actors who have so completely personified their roles, such as Downey and Jackman, have moved on.
