I don't think that was the only decision ... I am sure the loss of sponsorship was part of it but also it required a number of CMs to run and, while still popular, it's popularity had gone down a bit ... and mostly I think because it was showing its age and really needed an update so I think knowing they were going to have to put some $ into it they decided to gut it and do something different that would be an additional draw for people to do in addition to Galaxy's Edge
Also, the theme of the park is changing - it's no longer "about the movies" or the movie making process - it's about inserting you into the movies and being ultra immersive, so GMR didn't really fit that (I mean, you could make it fit the theme obviously, but I think that also was an added factor in changing it vs updating it)