I think people way overstate the whole "wear and tear" factor with Geno. There are examples of quarterbacks who were able to maintain consistently excellent play beyond the age of 35, but they are still a small exception, and the league continues to trend younger for their starting quarterbacks.
It feels like if "wear and tear" was a huge mitigating factor of whether a quarterback is able to keep up that high level of play, the league would be replete with talented career backups who transitioned late to quality starters as their contemporaries began to break down. I can't think of many examples of that. I suppose Rich Gannon would be a good one to point to. It wouldn't surprise me if some of you are able to think of 2-3 more.
I'm not completely opposed to the idea that his years riding the bench could help some in prolonging his career, but I really don't think it's as simple as taking the number of years he was a backup and subtracting it from his current age. On top of that, when he has been a starter, has Geno ever had the luxury of playing behind even a league-average offensive line? That kind of thing will accelerate physical deterioration.
Maybe Geno can be an exception, but I don't see the odds being enough in his favor where I would feel comfortable giving him a contract that keeps us on the hook for considerable money more than two years from now.