I think your theory is correct, but I'm less certain that Brady in Tampa this year at age 43 is the right case study. He knew he hadn't been the same guy since 2017 and needed a better roster to offset his decline. The Patriots had something close to that in 2018, but between Gronk leaving and Edelman now being 34, he knew he needed to move on.
The fact that the Patriots had to replace their QB on top of losing so many guys to FA and then to COVID kind of created the perfect storm for them to suck this year, but I don't think it changes the fact that Brady's declined a bit. 11 wins last year was a bit of a fluke given their schedule, unsustainable turnover margin/ST success, etc. We knew coming into the season that the roster was one of the worst in the league.
I thought he declined because he couldn't get his torso around to complete an out to the right side with the same velocity.
BUT--then I saw the game on replay Sunday night. He was zipping it on those outs. He was throwing lasers. And now I am reassessing.
In 2018, Brady was on fire during the playoffs. He played really well against the Chargers and Chiefs (at KC! in the freeze). He had a bad opening quarter against the Rams, but then again played really well. His offensive weapons? Edelman, Burkhead, James White. No other WRs. A hobbled Gronk.
So--he really didn't have a much better roster in 2019 than he did when they won it all in 2018. The big difference? 4 OL injured in 2019. Marshall Newhouse as his LT. This is why he kept throwing the ball away after 2 seconds.
When Brady left, it was still before Covid shutdowns. It was also as Julian Edelman was healing from broken ribs he endured during the season. Brady had every reason to think Edelman would be improved over last year (even though he had 100 catches). But Edelman has a bad knee injury right now--he shouldn't even be on the field.
Brady knew he was going to leave New England long before he assessed the talent on the Patriot roster. He decided to do it in the summer after the Rams Super Bowl. The Patriots offered him a 1 year deal at 20 million (Tyrod Taylor signed for $18.5 million shortly before that!). As soon as that happened, Brady put the house he built for $27m dollars up for sale. He was gone summer of 2018 no matter what happened in 2019. The Patriots told him he was on a year to year basis. He found someone that guaranteed him a longer deal. At the end of the day, he'll play for 3 more years (at least) post 2018 Super Bowl. The Patriots were projecting him for one more year.
This is the crux of it.
Personally, I think the Patriots should have definitely given him a 2 year deal.