The proof I have is common sense. Preston is a terrible coach that is also in way over his head in trying to be the GM of the team as well. He is creating a bad team and then handling it horribly as well.
It is pretty clear that anyone who had any desire to grow as a player would want to get as far away from the situation as possible.
There is nothing positive that a player can gain from hanging around a guy like Preston or on a team that is being run into the ground.