He is towards the end (not really debatable) , but I think he has some value depending on who is asking for him. But lets not act like his end is as bad as Peyton or Favre. Brady is a typical low risk moderate reward kinda passer whereas Peyton and Favre were High risk High reward type of guys. Also take in account the severity of his injury is nowhere near the punishment that Peyton and Favre were. So its entirely possible for him to still be a middle of the pack qb like he was this year.
But it gets to the question of what he is worth... As a Steelers fan I would love Brady on my team, but not at the expense of Bud Dupree. But for a team like the Chargers who have the potential of moving up in the draft to get Tua then I would probably pay a little more than he is worth to get Brady to be my steward while Tua heals and starts to learn. I wouldn't expect a Super Bowl, but a Wild Card game maybe not out of the question.
Then you have teams like the Cowboys in which you are married at the hip with Dak, and you know he isn't the answer. I think Jerry would be crazy enough to go after Brady to atleast win a crap division to shut the fans up. So Brady could mean a lot of sense there.
But in both in both of those scenarios Brady is given a weak division with better weapons than he has to work with, so more or less he can mask his age more in those situations while being a financial boom for those organizations and probably a steward to the organization's next face of the franchise. More or less Brady gets to play until he is 45 while the organization gets some heat taken off their front office. I dont think any serious contender should really go after him (unless you really think the Titans and Cowboys are serious contenders).