Time for this gem again.....
Remember when Walt Disney hosted a weekly TV show from 1954 until his death in 1966? Walt didn't just air the same episode every week. Whenever he did air an episode from a previous season, he would create new content or a new introduction for each episode. He used his program to create promotional synergy between his studio and Disneyland.
Also, long before there was the Walt Disney Theater in Florida, there was the Fantasyland Theater in Disneyland. It originally opened as the Mickey Mouse Club Theater in 1955, and stayed around until 1981. During that time, the theater wouldn't play the same film. It would change its programming to attract visitors who would come to Disneyland multiple times.
While I do enjoy the Walt Disney film inside One Man's Dream, I think 15 minutes is too short to cover Walt's life. If people really want to know about Walt Disney, I would encourage them to watch full-length documentaries like Walt: The Man Behind the Myth or the PBS American Experience special rather than a 15-minute tribute film (or better yet, read the biography on Walt Disney by Bob Thomas).
Honestly, I think Walt would be honored that a theater named after him features previews for upcoming movies made by the studio that he and his brother Roy created. As long as the films have some type of connection to Walt Disney (like animation) and not tainted with Marvel or Star Wars products, I'm fine with them using the Walt Disney Theater for sneak previews.
Also, the One Man's Dream film had been playing in the theater from 2001 to 2015. During that time, more and more people would skip the movie because there was nothing new added to it. Even when they closed nearby attractions like the Backlot Tour or the Animation building, the number of people who watched the film never increased. The movie previews were added to prevent the attraction from being shut down due to low attendance. If that weren't enough, they added a character meet-and-greet to the attraction which is bringing in more people inside but for all the wrong reasons. If anything, you should be more upset about a character meet-and-greet ruining One Man's Dream than a couple of movie previews.