By end we don’t mean that the MCU will be finished, but rather it will act as a finale for certain characters as we know them. Speaking to Vanity Fair, Marvel Studios president and executive producer Kevin Feige said that Avengers 4 will, “bring things you’ve never seen in superhero films: a finale.” He adds, “There will be two distinct periods. Everything before Avengers 4 and everything after. I know it will not be in ways people are expecting.”
There has been a lot of speculation about how Marvel plans on killing off/replacing some of the current characters in the MCU. For example in Captain America: Civil War, many expected the movie to play out like the comics where Steve Rogers is killed, but that clearly the MCU spun the story slightly differently. There is also the issue of contracts with the actors, in which some of them are running out following the release of Avengers 4.
In a way this is similar to how comics are approached, where both DC and Marvel have killed off characters and stories in apocalyptic endings, just so they can reboot them and start anew. That being said, we’re sure many of us will come to grow and love the new characters that will be introduced, and with Disney already planning 20 years worth of characters and worlds to explore, it really sounds like this is just the beginning.