There will always be a GMR. It may get periodic updates, upgrades, scene changes, etc, but the base ride concept will always be there. Why??, because it is the main ride about the movies in a park primarily about the movies.
I still question the placement of The Hat directly in front of GMR. I always thought the view down Hollywood Blvd was one of the most memorable vistas in WDW. Very much like MK and Main Street.
But I suppose any memorial to Walt takes precedence over all other considerations. WDW was his vision and long before Walt was in the theme park business, he was making movies. With the exception of Epcot, much of the rest of the WDW parks are connected to the Disney movies.
So in some way the placement of The Hat is appropriate, right in the middle of a place all about the movies. Walt, the consumate movie man, would approve.