Losing JMac will be a good thing. He's been objectively HORRENDOUS this season, allowing opposing QBs a ridiculous 134.8 passer rating when throwing at him. Williams - way too early to call him a bust. Gilmore and McCourty are free agents after 2021, but not before 2021. If they keep them both, they're still quality players. If they trade them, they can get some young players, or other players to fill holes, or draft picks to restock. They'll have to pay Jackson, which they should. He's elite.
RBs are the last position group I'd worry about. Harris is a stud. Michel is obviously more than a serviceable backup, and apparently isn't bad on special teams either. JJ Taylor might or might not be a decent replacement for White, but we know that BB always finds useful players at RB.
The OL...I agree with you that there are possible losses, but there's a lot of quality there, and they won't lose ALL those guys.
The DL and linebackers are an issue, but there's some decent players there. Pass-catchers are an issue too, but honestly, they add one really good WR and the other guys all get much better. So think of it this way. Byrd is a useful #2 (with a good QB) but a bad #1. Meyers is a useful #3 but a not-so-good #2. You add a true #1 WR and you improve all three WR positions in one shot as Byrd and Meyers get bumped down a slot. Lots of WRs available this offseason. Same with TE. One guy I'd love to see them snag is Arnold from Arizona. He wouldn't cost too much, and his stats aren't lights out, but every time I see the guy, he's making plays. UFA after this season.
So it's possible to upgrade rather significantly with just a few key moves. The QB spot is the issue, though as I pointed out, there could be legit options there too.