It is not clear that running is bad for the knees. Recent studies ([1], [2]) find no evidence of any adverse effects, rather the contrary (lower incidence of knee osteoarthritis in the running group). One possible explanation is that the runners were less likely to put on weight over the years, leading to lower day-to-day stresses on the knees.
[1] Eliza Chakravarty et al. "Long distance running and knee osteoarthritis: A prospective study," American Journal of Preventive Medicine, 2008, 35(2), 133-138
[2] David Felson et al., "Effects of recreational physical activities on the development of knee osteoarthritis in older adults of different weights: The Framingham Study," Arthritis & Rheumatism, 2007, 57(1), 6-12
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.
its far from crystal clear - each body is different, what builds one breaks another one (even if fitness levels are similar). genetics is not fair.
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
[1] Eliza Chakravarty et al. "Long distance running and knee osteoarthritis: A prospective study," American Journal of Preventive Medicine, 2008, 35(2), 133-138
[2] David Felson et al., "Effects of recreational physical activities on the development of knee osteoarthritis in older adults of different weights: The Framingham Study," Arthritis & Rheumatism, 2007, 57(1), 6-12