Belichick is a guy who needs to have a lot of control. The Patriot Way is a dictatorial one, and he hasn't compromised with anyone in the last 20 years.
I can't see him getting that level of control after his last 10 or so drafts all flamed out. He would be married to a GM he most likely doesn't agree with, who just hands him players and tells him to make the best of them. BB can still call a defense on par with anyone, but he would be a glorified DC in his new team, and I don't think he's ready for that after being Emperor for so long.