BB knows he has a great chance for next year. Doesn't want it ruined by having Brady go down and an unreliable backup. Still think there's a good chance he trades JG wants he gets a look at Brissett and how he's developed. The fact that Cleveland hasn't taken a QB yet sort of supports that notion.
Also, and I can't stress this enough, when Schefter said that the Pats weren't trading JG, he said he'd still be a Pat this summer. Why "this summer"? Why not next season? We know BB isn't overly bothered about this draft and we know Cleveland aren't a Jimmy Garropolo away and that they've stockpiled picks. I could see a JG trade to Cleveland in the summer if BB is comfortable with Brissett as a backup.
Or they really do see JG as Brady's successor. QBs are everything in the NFL after all.