The list of players that are no longer on the SBU Roster have left the program for an assortment of reasons; walk-ons who were never going to play, academic issues, injuries preventing them from playing, discipline issues, etc.
Jack Coan has not been guaranteed the starting job in fact the 5 Star QB that signed was guaranteed that he will have a chance to be the starting QB if he out performs everyone else so don't assume Coan will be the starter. Also he played in a lot of games as a true freshmen and sophomore so he may only have 2 years and not 3.
I also have heard the transfers that their after are like players of the caliber of Casey Williams who came in last year and did a great job.