Tom left for 2 reasons, 1, enough money that he felt respected, and 2, enough weapons that he didn't feel like it was a grind, where he kept having to be the bright spot on an otherwise not-elite team. Subtracting TFB from NE put TFB in an immediately good situation, and, well, we know what it did to NE. It's also in a big stew pot of NE-specific 2020isms almost as fun as the real life 2020.
We responded to the virus as bigger than football -- which I think was smart -- so it was, in terms of football results. Many impact players took the year off (disproportionately so.) Cap space made it really hard to scrap and compete for other talent. We ended up with a depleted roster and a QB who ended up playing up to the very low expectations we put on him, among other ills.
No excuses for BB (he said, after a paragraph of what I'll call ahem "reasons"; we'll see how it all shakes out post-Brady.) But I don't think you can say after 2020 that we know that Belichick wasn't that important. By the same token, all we've established is that Brady can still bring it at an elite level against Detroit -- although if it's not against NE, it's fun to watch him succeed. Point is, he made the playoffs, he didn't just win another SB.
Now if TB goes deep in the playoffs, you can say TFB won the divorce, for now. If NE turns into the Browns of the 90s, I guess maybe BB wasn't that important -- just a decent coach, right place, right time. We'll see if Coach/GM BB can return to form... but I don't think enough data's in right now.