According to The Hollywood Reporter, Kevin Feige explains that they have all of the key players in place at least as far as Marvel has movies have been planned out. This would extend out until around 2019 and, after Ant-Man, includes ten movies (Captain America: Civil War, Doctor Strange, Guardians of the Galaxy 2, Spider-Man, Thor: Ragnarok, Avengers: Infinity War—Part 1, Black Panther, Captain Marvel, Infinity War—Part 2, and finally Inhumans). But as you look to the future, Feige continues with:
But one day, it will be a combination. It will be a combination of choosing which characters to continue on with and explore and which to bring in, as we're doing with Captain Marvel and Black Panther. And certainly to recast in some way or another – as we've already done in certain cases. Certainly many other franchises have reached that milestone before we have and done that with great success.
From this statement alone it looks as if Feige is definitely not against replacing certain Marvel characters, if need be. Comic characters are dying pretty consistently, but as far as the Marvel Universe goes, we haven’t really seen the death, as of yet.
According to Cinemablend.com, There have been lots of rumors about Captain America possibly kicking the bucket, especially as Chris Evans’ contract draws to a close (though he has talked about being willing to re-up, much like Robert Downey Jr. has already done, and come back for more). Steve Rodgers isn’t he only person who has ever been Captain America. There have been tons in the comics, and the MCU already includes two individuals who have donned the red, white, and blue outfit, Bucky Barnes (Sebastian Stan), better known as the Winter Soldier, and Sam Wilson (Anthony Mackie), who plays Falcon. This is one way to "recast" without the audience having to pretend it didn’t happen.
Courtesy of Cinemablend.com