I'm inclined to agree. Brady is already way off the deep end of the Bell curve. I can understand fans wanting and convincing themselves into thinking it will last forever but players and owners need to be smarter than that.
No matter how long he goes, at this point you take any athlete at that age year by year, no more than that.
Other posters have pointed out that Brady is still playing better than ever, and even more remarkably has remained injury free, and they have theorized that Brady will stay at a top level for several more years. You have made the case (in a reasoned way, in my opinion) that it is really unknown how long Brady will stay at a top level, as he is already at the deep end of the bell curve.
The fact is nobody really knows. I have hopes that Brady stays at a top level for several more years. If he played well to 45, that would be FANTASTIC!! But, as you point out, there is a long history of athletes that have declined rapidly in their 40's, become injury prone, or lost their will to do what it takes to keep their body in shape (although I have trouble believing that would happen to Brady). There are no certainties in life.
I think it is clear that BB has has been preparing a backup plan (as he always does) in the event of a Brady decline / injury for years, otherwise he wouldn't have spent so much draft capital on QB's. I'm sure that year by year BB has been absolutely delighted that Brady has stayed at that top level, and remained injury free. I actually think he will keep Jimmy G. this year, I think that BB will want to have a top QB backup for his 40 year old quarterback in case of injury, if nothing else.
Nevertheless, as long as Brady maintains anything close to his current performance, I think he will remain the starter for the Patriots. I frankly can't imagine the Pats trading Brady, I think the fans would completely revolt (I certainly would).
Once Brady finally suffers a major injury or takes a big step back performance wise (and, being the perfectionist that he is, Brady will be the first to recognize that), I hope that he will go the Bruschi route, decide to retire with dignity and honor as a Patriot for life, and stay on with the team in some capacity. That is my dream, and I'm sticking to it.