FC was his origin story and we saw him grow and mature as he assembled his first group of students. The events of the movie end with setting him on the course to eventually be the man we would know in the future. In the interim between FC and this, he had formally opened up the school to more mutants and had become a teacher to gifted youngsters . He was officially being called Professor. This was all explained by Young Beast and we even saw flashbacks. What this movie was undid all of that to reboot the status quo to get him to where he was before all of that and redo setting him back up to reopen the school, gain more students and become Professor. He had already done that!
When FC ended, did you or anyone else REALLY feel "oh I wonder how he becomes the Professor of the school, that we'd know decades into the future?" FC set things up perfectly and told a good origin story, which I felt was pretty clear. There really wasn't anything that needed to be expanded upon.