If you consult most orthopedic bodies of authority, they are incredibly fatalistic about the prospects of adult joint cartilage, stating definitively that it does not regrow. In fact, they’ll usually throw in something about how it “can’t” regrow due to the fact that there is no direct blood supply. Umm... yeah, about that. No one ever said it needed a blood supply. Cartilage uses a diffusion process to obtain nutrients. It would be gone in a single month around the age of twelve if it couldn’t obtain any nutrients.
Beyond anecdote, studies already prove that joint cartilage does regrow: https://jamanetwork.com/…/jamainternalme…/fullarticle/410037
This includes hips, knees, shoulders and the discs between vertebrae in the spine. In fact, even the once-believed-to-be impossible disc herniation, it turns out, often spontaneously heals: https://www.sciencedirect.com/science/article/abs/pii/S1878875011004992
Cell turnover rates in the body are incredibly well understood. Anyone who insists that tissue has a one way ticket of no capacity to recover or heal is ignoring the entire subject of biology. There is only one situation which will irrevocably degrade joints, and it has nothing to do with age or injury or anything people tend to blame. Rather, joint tissue degrades via unloading: https://www.nature.com/articles/s41536-017-0016-1. That's right. When you stop putting pressures on cartilage, it has ZERO capacity to even maintain its current integrity, let alone get better. We've known this for a while, thanks to astronaut studies, wherein young, fit, perfectly healthy and athletic people "age" their bones, muscles, and other tissue the equivalent of 50 years from just a few weeks of weightlessness: https://phys.org/news/2010-08-astronaut-muscles-space.html. Even children whose legs are cast have loss of knee cartilage. It's a fully known phenomenon.
This is why we have to load structures, especially when we fear they may have accrued damage and wearing. The loading restores them. Why? Because the MECHANISM. We understand the MECHANISM. Diffusion requires pressures. As we take away pressures (inactivity, decreased loading, etc.) we take away the known process to manage cartilage. As we add pressures (strengthening, movement, loading, etc.) we enable the diffusion process. Does this mean any old pressure will do? Of course not. Repetitive pounding on the same angle of the joint will likely outstrip the regenerative rate.
Again, this is where controlled strength training defeats all other modes of exercise. You place pressures in a controlled fashion on the body at different angles, with variant tensions. Thus, diffusion. Whereas walking, distance running or cycling will tend to hammer the exact same spot over and over. And cartilage does have very slow cell turnover rates. So frequent high volume repetitive activity will likely degrade or accelerate joint degradation.
We’d expect, therefore, to see a nice bell curve of joint degeneration based on activity. At one end (underuse) there will be a high incidence of cartilage loss, strength loss, and other musculoskeletal loss. At the other end (overuse) there will be a high incidence of cartilage loss. At the underuse end, we’ll find a rampant degeneration because inactive people will have other metabolic factors impeding the repair of all tissue. In the middle, we will find some asymptomatic people. Look it up. Every analysis confirms this expectation.
Likewise, reducing body mass (specifically fat mass) will reduce the chronic overuse burden on joints. Thus, on summary papers, researcher look at this issue and find that weight loss improves joint defects: https://jamanetwork.com/…/jamainternalme…/fullarticle/410037. The individuals who have improvements in cartilage defects included some sort of physical activity, but not excessive repetitive activity. They still abided by diffusion, but didn't overwear the cartilage. They improved nutrition, such that cells can actually repair themselves.