So far, Feige seems committed to the idea of successors carrying the mantle of a hero rather than reboots. And avoiding a recast if possible. With the multiverse factored in, the MCU has the potential to combine the best of both worlds.
1. The mainline MCU universe stays a place of weighty history and generational storytelling. Characters pass their name and iconography down to new people over time.
2. "Founding" characters, like Steve Rogers or Tony Stark, can still make occasional appearances thanks to the multiverse. Played by whichever actor is most feasible at the time.
They'll probably be able to still use Chris Evans/RDJ etc in the movies via de-aging CGI which they've already used in Marvel movies. By the time these actors are dead the technology will likely be so advanced that they can still use their likeness in perpetuity.