That proves that Bill can be decent without Tom. But the issue is that long term
The main problems are:
Other historically great coaches have actually had some deep playoff runs (Landry made 3x NFC title games, for instance). Bill has just 1 playoff win, and 0x division titles. The difference
And Belichick's philosophy just doesn't hold up over a long period of time without Brady. If you can't draft WR, don’t like paying a premium for star players, and you aren't particularly liked by a lot of your players, and you make ego-based decisions, then you're generally asking A LOT out of your QB both on the field and in the locker room.
Brady made that look more effective than it should have been.