His improving over the last three years is due as much to the team making improvements around him as anything else. He hasn't declined appreciably. But he wasn't over 40 then either. I do think he's lost some arm strength. He's putting much more of his body into his throws than he used to. And FWIW, he agrees with me on that. He's said he has to be even more careful of his mechanics and involve more of his body in his throwing motion.
It is what it is and we'll see. I love TB and would love if he somehow managed to magically keep himself at his pre-40 condition. But I don't believe in magic. Nor do I believe in the efficacy of avacados and concussion juice. I'd like to see TB at 50 not crippled or with cte or alzheimers.
And that's the thing -- when you talk about deterioration of a very cerebral player, the stats are the last things to go. Curt Schilling is a bonehead in terms of life skills but he was one of the smartest pitchers I ever watched, and he was able to exploit his remaining skillset to pitch at a very high level deep into his declining years, having to stop only when his body couldn't take the strain at all anymore. Nolan Ryan got even further simply by having so much sheer natural talent that it took years for it to all go away. Both of them managed to be effective despite severe decline, simply by knowing how to deal with the talent they had left. Brady will do the same. I think he's been doing it for years already.
Tom Brady is very smart about knowing his limits and finding ways to get things done. The first sign of deterioration is not going to be Brady not getting it done on the field -- it's going to be Brady making several minute adjustments to how he gets things done, to adjust for his aging body and take best possible advantage of his remaining abilities. I think we've been seeing that for at least the last 3 years, and Bill Belichick has been aiding and abetting that with roster adjustments that play to an accurate assessment of what Brady is at any given time. BB has just done a sterling job of building the roster around Brady's current strengths and not getting stuck on the guy Brady used to be -- so that he's not getting exposed and his physical decline (everyone starts declining physically starting between the late 20's and mid 30's) is being intelligently managed both by the player and by the coach.
(as a side note, I think TB12 got back on Superbowl form after several seasons of what could best be called wilderness years in part because the roster improved, but in part because he finally recognized he was in decline, reassessed his stock, and made adjustments to his playing style that returned him to an elite level -- it can take some integrity and personal courage to admit you've hit that stage of your career and it's to Brady's credit that he stuck the transition so well)
It's telling that BB has really worked hard to beef up the roster this year, I think we'll find BB making more and more aggressive moves to make the most of Brady's remaining seasons and keep him playing at a high level as long humanly possible.
That does not mean that he isn't declining. At that age everyone is declining. It just means his decline is being managed intelligently so that he is not asked for more than he is capable of at any given time.
There will come a point though when continued adjustments will meet with diminishing returns. There will come a point where the decline can no longer be managed intelligently. I think we're actually fairly close to hitting that point, which is why BB brought in a premium wideout, something he almost never does, in this offseason. That's the last throw to keep Brady at an elite level. Cooks should buy Brady at least an additional year or 2. Maybe you can get one more jolt if BB invests in an elite running back to take the strain of the offense off Brady's arm. Maybe after that you can be the team we were in 2001-03, with Brady as a field general managing the offense and making short, intelligent throws while other facets of the team carried a large portion of the load. But eventually this ride ends, and there's NWIH it takes 7 years.