It takes a special mentality to understand that you will be a back-up. And I don't see how he fits McVay's offense. He's not a particularly accurate passer, has a huge ego, and is used to being "the man". Not saying that he isn't a talented guy, or he isn't a good guy. But some guys are wired to be back-ups and others aren't. And he'd have to come hat in hand (as Bortles did) to work behind Goff.
Plus...... the first bad game Goff has the fans would be clamoring for Cam. It is just a bad idea all around.