It's tough, because Belichick is a great coach. But the issue is that even if you play nice and only start counting since 2001... his overall accomplishments in that sample don't point to him being "THE GOAT". 11-5 with Matt Cassel and 7-4 with Mac Jones are the highlights in that sample, but that is that more impressive than freaking Rex Ryan taking Mark Sanchez and the New York Jets to the AFC Championship or bum ass Matt Nagy taking Mitchell Trubisky to a 12-4 record and playoff birth.
In short, he needs to win a ring, because A) Belichick is such a unique coach in that we have a decent sample size before, during, and after Brady, that isn't that great, and B) we've seen Brady win a ring without him. We've rarely seen that before with a coach QB duo.
To finish up, Belichick is a great coach, but him being the GOAT is still debatable, because I think it has become clear that his success and his system is very contigent on a very specific QB archetype with a high floor. One could say any coach needs a good QB, but then that would hurt Belichick's case even more. We've seen other coaches achieve things with QBs that aren't even that good at all. Belichick had Brady for 20 years.
Based on how good Mac is, I think Belichick has to win at least a 1 ring with him.