I don't think it's true that Brady "isn't very good."
Give him time to throw and solid receiving options, he'll make plays. He can be relied on to drive you down the field. If he trusts his receiving options and isn't forcing it to much too one guy (Edelman), he won't throw many picks.
Put him behind a piecemeal offensive line and give him only one or two guys he trusts to catch his passes and he's not going to look great.
Brady can't elevate a team anymore, but he's still good enough not to drag down an otherwise elite team.
"Done" to me is Peyton Manning in his last season with the Broncos. Even though they won a title, he had a noodle arm and only a historically amazing defense and a lot of luck got them to a title. Brady has more juice at this point than Peyton did back then.
He has a bit more than Manning did that season, but yesterday he wouldn't throw the ball downfield. He was way off on several passes, some even to open guys. I just don't think he has it anymore. He could probably play the game manager role, but that requires a commitment to the run, something New England hasn't done in a decade. And if you are going the game manager route, why not just move on.
I think he's hurt.
Even so, I've already said that I think it could make sense for both sides to move on.
Brady should go to a team that is just a competent QB away from being a SB contender.
The Pats need a guy who can make all the basic throws while also having the ability to pick up yards or even TDs on the ground when the defense leaves an opening.
My perspective on the Patriots' receiving core is not that it is terribly lacking in quality options, except at TE. The lack of good pass catching TEs is a big issue that I hope they address this offseason.
The receiving core is not that bad. Meyers, Harry, and Dorsett have all shown that they can catch balls and make plays. James White has been strangely underutilized this season. Burkhead can catch passes as well.
The issue is rather that Brady doesn't trust hardly any of these guys other than Edelman. This results in Brady over-relying on Edelman, especially when he has less time to throw because the offensive line is a mess.
If there's one complaint / weakness I would mention about Brady over the last 10+ years, it's that he has a tendency to lean too much on guys that he trusts (Gronk and Edelman especially) when things get tough. But Brady is the GOAT, irrespective of whatever flaws he has.