All this. Brady might be able to play, and play fairly well, to 45, or so. But if he keeps on getting hit (something that didn't happen much, until this year) he will get injured. How badly injured? Who knows? As has been mentioned, no matter how good you condition, bones break. And, as we age, we take longer to mend.
The Pats have typically had a QB in the fold who could play, reasonably well, for a few games---even when they had Mallet, but they were lucky and didn't need to start him. I'm guessing they find another decent backup, for next year. This year however, they are playing with fire, IMO.