As you get older things hurt. The wear and tear one has put on their bodies adds up.
For an extreme example, look at professional athletes. Drew Brees still had the desire and drive to play, but his right shoulder had simply given out. The final result of an injury over a decade ago. He cannot throw right handed anymore.
A lot of these guys require knee replacements after they retire. Permanently disfigured fingers and toes. Etc.
So would it be fair to say that the increase in cortisol is also a protection mechanism? Since older bodies are less able to heal injuries the cortisol increase acts to prevent behavior that might damage them?
I'm saying that it's quite possible not related to cortisol at all. Or possibly you have the cause and effect backwards. Because of all the wear and tear, we become more cautious about certain activities, which causes more stress, which then causes us to produce cortisol.
to be fair, op is correct that the majority of people that get replacements fall in the sedentary demographic and don't have a history of intense physical activity. metabolic issues and pro-inflammatory factors, not mechanical stress, are for many people the primary driver of osteoarthritis and soft tissue issues more generally.
But people who play professional sports do get joint replacement at a higher rate than even the general population.
Just because there are more of the "general population" doesn't mean that the subset that are "professional athlete" isn't doing it at a higher percentage.
And there are other, non-extreme, things that also wreck the body. Working in the trades: plumbers, electricians, HVAC, construction, etc. will often leave you with bad backs and bad joints.
And yes, living your life at the extremes is going to have extreme effects on your body. Sedentary is the other extreme of professional athlete.
But even still, as things hurt more, we will be more cautious about endangering ourselves. Or even wondering if we are capable. If I can't walk without a limp, I'm sure as hell not running from danger. That's not cortisol driving our behavior, that's our behavior driving our cortisol.
I don't think anyone disputes that high-level sport can make you more vulnerable to particular msk issues than people who have led a less physically extreme existence. Overuse is a real thing! The relationship between cumulative load and tissue health is more complicated than you acknowledge, however. High cumulative load over life can be protective. Professional marathoners frequently have better knee health than sedentary people, yes, but also age-matched recreational runners, which is not what you'd expect if extreme load was straightforwardly harmful in the way you seem to think it is.
Regarding your comment on cortisol. Plausibly the relationship between cortisol and behaviour here is bi-directional, as is common with hormones. High testosterone influences me to go to the gym and my workout influences my testosterone, etc.
N == 1, but when I bought a Garmin watch and started doing 10 000 steps every day, all my lower limb joint problems disappeared in two months or so. Nowadays I don't even know which knee used to hurt worse (I am 45).
Mind you, I wasn't completely sedentary before, but I guess that Garmin made me move about 40 per cent more.
We don't really know if its from "wear and tear" or from steroid abuse which weakens bones and ligaments, artificially high bodyweight, injuries during practice and in the gym.
For an extreme example, look at professional athletes. Drew Brees still had the desire and drive to play, but his right shoulder had simply given out. The final result of an injury over a decade ago. He cannot throw right handed anymore.
A lot of these guys require knee replacements after they retire. Permanently disfigured fingers and toes. Etc.
And we all go through a version of that.