Lots of speculation, and certainly the Tampa coaching staff leaves a lot to be desired, but I think an under-discussed element of Brady's slippage is his arm strength.
It's so subtle, he hasn't lost a ton of velocity, and it certainly didn't happen overnight like with Manning so we don't notice the big drop, but I think his arm has been slipping a bit since 2016'ish. You get a 2-3% drop-off per season, aggregated over 4-5 years, and here we are.
What does this actually mean? Downfield throws are more challenging; his feet have to be perfect set and he needs a significant 'step-in' or the ball is underthrown. Throws outside the numbers are tougher to come by. Any kind of pressure in Brady's lap that affects his ability to step into throws will considerably impact the velocity of the ball. The overall margin for error is much smaller because the ball is arriving a split-second later.
Go watch Brady highlights from 2007. Or hell, even 2010. The ball was a rocket coming off his hand. He could flick the thing from the pocket, under pressure, without a perfect step-in, and the ball came off his hand hot. It just looked so much easier for him to throw the ball - he was an extremely talented passer to go along with his work-ethic, preparation, and intelligence. Now, he simply doesn't have the arm talent he used to. What's the drop-off? Probably 5-10%, maybe 15%, certainly not significant, but that split-second margin can make a really big difference.
Is he still the most intelligent QB in the league? Absolutely. Is his work-ethic and preparation still unmatched? Surely. Is his timing and decision making still pretty darn good? Yeah. Is he still a top-10 QB in the NFL (top-5 on a good day)? I think so. But he's 43 years old and has thrown thousands of NFL passes ... the arm just isn't what it used to be.
I think we really underrate the impact of lost arm strength - the ability to flick the ball and get it anywhere you want and on time is huge; when that goes, the margin for error shrinks and begins to place a spotlight on decision-making and timing, because you don't have a live arm to open up the margin for error. It's not just the arm strength itself -- it's the trickle down impact that has on everything else.
I still think Tom can win and compete at a high-level in his current condition, but it won't be under a myopic coach like Arians, passing the ball down the field 40 times per game. Setting aside their offensive talent, from a coaching/scheme perspective Tampa was one of the worst fits for Tom at this stage of his career. He should've stayed in New England, accepted 2020 as bridge season, and got ready for a final 2-3 year push from 2021-2023 with their salary space.