I wrote 'extensive running'

Is it 20-30 miles/week over a long period, which you claim is already way too much for the majority? If I read study [1] correctly the runners were running 3.5 hours a week at the beginning of the study, which would be around 20 miles/week. That same group had a lower incidence of osteoarthritis than the control group.

Do you have any evidence to back up your claim?

our bodies require challenge, otherwise they get weak and fragile. challenge too much, it will break. this point of diminished return is again different for each of us, and varies over time for same person.

BUT - if you give it enough time to fortify itself - I don't mean muscles, they grow faster than rest of the body. I mean when bones, tendons, ligaments and generally joints catch up (ie for climbing it takes years of gradual progression to train your fingers), they hold the body together better (knees, feet, spine etc.) and can manage stress put on it, even till high age. I have a friend who is 80 year old mountain guide and he still climbs easier stuff.

But to find the balance, the edge of benefits vs losses that is never stable and unique to each of us is very hard

Bad knees can be the result of many, many things, including an injury which may have been the result of twisting your knee rising from a chair.