I'm sure they know they'll have to somewhere down the line, but I think the ultimate plan is to try to develop these new heroes to supplement them first. Though, Feige has always said that he viewed Tony Stark as a character who, like James Bond, could continue on with various sequels through recasting, so I'd assume he shares that ideology with other heroes.
Who knows though? Theoretically, they could give RDJ a nice piece of the pie to continue on in a role similar to SLJ's Fury, where he could be present in a limited capacity and suit up for The Avengers films. (Not like it's too stunt-intensive since Iron Man is largely CGI'd.) Hemsworth and Ruffalo have shown a willingness to keep on with their characters for the foreseeable future. Evans is tough to say because, should they choose to, the comics storylines somewhat set them up to replace Cap with Bucky or Falcon. It's not like they couldn't recast Evans though. He's been a solid Steve Rogers, but he's probably the most "recastable" of the three. Seriously. Throw Armie Hammer in there. No one's going to complain.
Still, eventually, you're going to run out of villains/storylines to adapt. It feels like there's some finality in sight to Iron Man and Thor's individual stories. They've said they have movies planned out until 2028. That could take them through an Avengers 5 or 6. At that point, you're 20 years out from Iron Man first hitting the screens, so maybe it's an opportune moment to pull a hard reset on the the whole MCU. I'm sure they wouldn't mind starting over with the confidence that they know they have some long-time security, where Phase 1 was largely on the fly and not really tied to any long-term goal beyond getting The Avengers together.