For sure, I would also rather have the greatest QB of all time or the next great QB .That's just it though. It is a cumulative effect. It's the same reason RBs simply do not last that long on average. So I think you're correct, Cam did not start to deteriorate until about year 4 or 5 or so. Really, since his Super Bowl year it has been a decline in health. He should be in the prime of his career.
I'd rather have a Brady than a Mahommes. Even though Brady is on the opposite end of the spectrum, a QB that stays in the pocket has a far better chance of a 15 year career. Mahommes may adjust his game, and to his credit, he does tend to get down before taking hits, so maybe he's a bad comparison.
But I do agree. The Panthers put a lot on Cam, especially during the Super Bowl season. He took a ton of hits both in and outside the pocket and did a lot of QB sneak type stuff IIRC. He's taken a ton of punishment.
My bigger point was just that I wouldn't bet on injury being the only way Mac gets on the field. Unless Cam plays like he used to I think his play and losing games will get him benched at some point.