great legends of BYU basketball? If he had stayed all 4 years, he would have finished as BYU's all-time leader in points, rebounds, and blocked shots. Probably would have been All-American at some level. Possibly win a conference championship or two - which would be especially meaningful because BYU has yet to win one in the WCC. That's a pretty big deal, IMO.
He seriously could have been one of the most famous and popular athletes in the history of the university. And Italy was always going to be there - leaving early last year didn't improve his chances of landing in Europe at all. He hurt the program and did nothing to help himself. And a lot of fans know that, and they are (understandably) angry about it.
I do agree with you that he's gone now, never coming back, and BYU fans should probably let it go. But saying that "there was literally no reason for him to stay" is totally ridiculous.