The entire source of PC fans' anger is this, that he demonstrated clearly and definitively that PC is only a stepping stone.
Everyone they hired after Gavitt left to launch the BE was either incapable of sustaining any success (Mullaney 2.0 [too old], Chiesa, Welsh, Davis) or willing to jump once something better came along (Pitino, Barnes, Gillen).
Cooley was supposed to be different. He was supposed to be one of them (he was born and raised in Providence). With him leaving for greener pastures, they were slapped in the face with the prospect that they may in fact be doomed to being little more than a pit stop on a coach's resume.