At that time only a few pro vert skaters would have had the ability to throw 720s, yeah. Nowadays a good number of ams can too.
The rarity of seeing a 720 or above has as much to do with the fact that most skaters don’t skate vert - instead skating street or smaller transition - as the trick’s difficulty. Outsiders tend to imagine large spins are the holy grail of skate moves but almost all skaters aren’t interested in them for aesthetic reasons among others.
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.
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.
To a limited extent, we do. There are some studies in which research groups try to enhance the regenerative capabilities of mammals in a way that resembles the regeneration seen in amphibians. See the journal article linked below, as an example.
It is important to keep such research in perspective. I see it as "proof-of-concept" research - studies with biologically intriguing findings that might amount someday to something effective, treatment-wise. For now though, regenerative therapies are underwhelming IMHO.
The study of soft-tissue healing in animals besides humans has definitely informed what and when medical interventions are used in MSK healthcare. But even in the easier case of mere healing, most of the good animal model studies have led to only a kind of negative medical progress — i.e. they’ve given us reason to stop providing a medical treatment we previously thought might be beneficial.
Sticking with tendons for example. Docs used to be much more willing to give corticosteroid shots as a treatment for chronic tendinopathy, but more and more animal and clinical studies show that corticosteriods have a catabolic effect on tendons in the long-term and so should be used only when the short-term benefits are expected to be very large.
Medical interventions where regeneration in the context of injury is the specific goal (e.g. PRP and stem cell therapy) have a very mixed reputation within MSK circles, I believe — many clinicians seeing those treatments as basically a sham at this point.
The rarity of seeing a 720 or above has as much to do with the fact that most skaters don’t skate vert - instead skating street or smaller transition - as the trick’s difficulty. Outsiders tend to imagine large spins are the holy grail of skate moves but almost all skaters aren’t interested in them for aesthetic reasons among others.