And I remember Favre looking hobbled at 39, Elway limping around during his last two SB winning seasons, and Manning barely able to throw the ball. I could go on. I don't remember any great QB looking so spry at 39 like Brady does. I don't remember any of those guys devoting their diet and exercise routine to staying heathey the way Brady does. I don't doubt that Brady could play at a high level at 45. I hope to see it.
Manning's decline was ugly, although I wonder how much injuries had to do with it. Elway missed a few games that last season, but he was still playing at a pretty high level. He finished the season with his highest career RAT of 93.0, and had a combined 49 TDs vs. 21 INTs those last 2 seasons.
Favre's 39th year was with the Jets, and he was playing hurt after his 11th game. Look at the splits before the injury though. 8-3, 70.6% completion, 20 TDs, 13 INTs, 94.1 RAT before vs. 1-4, 56%, 2 TDs, 9 INTs, 55.2 RAT. But he would bounce back to make the Pro Bowl at age 40. The wheels came off at 41, but part of that was Favre announced it was his last season so I doubt he was really doing the things he needed to do to be at least a bit more competent.
So it really does come down to health more than anything, and I think Brady can actually do it. For one thing, he's not relying on extreme athleticism like Rodgers, who seems to have lost a half step which is juuuuuuuust enough to return to being a mortal.
Brady's game is intellectual, and the bread and butter of the offense doesn't require a huge arm but rather accuracy, which won't fade as quickly as arm strength. He also likely won't face a major coaching/system change like Manning did at the end. I think Manning's end would have been ugly regardless, but the system change probably exacerbated the issues.
45 might be pushing it, but I expect him to play out his current contract at a high level. And many people have gone broke betting against that 6th-round, 4th-string QB. I won't be betting against him.