Devils advocate response.
Quincy wanted to explore his professional options with respect to the NBA. His timetable didn't line up with Syracuse's vis a vis recruiting, so the coaching staff actively went out and recruited two frontcourt players [Swider / Jimmy] before Guerrier's final decision came down. I think it's one of those damned if you do / damned if you don't situations for the staff -- if they don't bring in quality replacements because they wait to long, they're left holding the bag. And if they act swiftly and strike while the iron is hot, then they "push" Guerrier out. Unfortunate situation, with the timing, IMO.
He could certainly have chosen to return, because as you point out he would have been a returning starter. But he also might have felt that the team is structured about Buddy getting shots, and that might not have enabled him to be featured in an advantageous way.
Quincy is also a helluva college player, whether he eventually emerges as a legitimate NBA prospect or not. But there will be high major interest if he actively pursues a change of scenery -- make no mistake about that.