Belichick had one playoff appearance and a 37-45 record with the Browns and a 5-13 record with the Patriots before Tom Brady became a starter. Fact. In comparison Pete Carroll went 28-23 with Bledsoe and the Pats and got canned. IMO Belichick was on the same trajectory, and if he hadn't drafted Brady, he likely would have been fired a long time ago.
TB12 has succeeded with every OC he has had.
Tom helped create the "system" we have now - he is a hard worker, and you can bet he has major input and insight into what to do, and what works.
Belichick has always been a great coach, and has also become a fantastic talent evaluator. Garoppolo is a great prospect IMO, and could be a future Pro Bowler. BB has grown and improved, and has filled in the cracks after some earlier poor drafting that led to some very bad defensive play in the 2008-2012 seasons. He deserves a ton of credit. But without Brady I don't think he lasts long enough to get as good as he is.