Personally, as simply a fan of the films and never having been invested in the comics or its characters, I'm really not fond of the recasting idea. If the actors grow too old or don't want to return, I think writing them off (whether by simply retiring them or killing them off) would be my choice. As for their superhero alter egos, they could either retire them along with the characters (at least for a while), or have them be passed on to another character (Peter Parker to Miles Morales, Steve Rogers to Bucky Barnes, or what have you).
I understand that for most fans, the super hero alter egos are tied to the original characters in a way that it would seem like blasphemy to have them disappear from the MCU, but I think one problem with the MCU that the comics universe doesn't have is that time basically passes in real time. For the most part, time in the MCU has been running parallel to the real world. As we get older, the characters get older, and Tony Stark isn't eternally youthful. So would it make sense for him to be recast every 10 years, hovering around in the 35-50 age range?
And while comics fans may be invested in many of these personalities, before the MCU popularized these characters, the general audience were never too familiar with the people behind the masks, let alone the masks themselves. Sure, many knew of Peter Parker/Spider-Man, but did they know that Steve Rogers is Captain America or that Tony Stark is Iron Man? They may not even have known that an Iron Man existed at all. I think that if new characters that are likeable and charismatic are introduced as "replacements", the audience will latch onto them. And I think Marvel has built up enough goodwill that audiences will be willing to give them a chance to show themselves worthy.
I mean, there's no doubt that losing RDJ/Tony Stark would be a huge set back for the MCU, but once they lose RDJ himself, I don't think a recasted Tony Stark would give the MCU any clear advantage over another character taking over the Iron Man alter ego. If anything, it'd be a jarring change and it would be much harder to accept that a completely different person is supposed to be the same character. I'm aware that Feige has said that the current plan is that they want to go with recasting, so I'm just stating my opinion here. But personally, as a continuity nut, I've already been a bit jarred by the recasting that they've done for Bruce Banner and Rhodey, and those were relatively minor characters compared to Stark, and both were replaced after just one film. RDJ has now been Stark for years in multiple films; I personally think that would make any recasting all the more jarring.
TL;DR. I'm not excited about the recasting idea.