Yeah. Osteoartritis is a condition that has virtually no good treatment.

Bioengineer here able to chime in with niche expertise. Cartilage is and has been a holy grail in biomedical engineering but is very difficult to grow and transplant. There have been some successful neo-cartilage projects but integrating that into a defect and successfully integrating with host cartilage is the problem. The boundary of the defect actually has electronegative components that actively oppose integration of host and transplant (synthetic) cartilage. I think the most likely solution won't be a tissue-engineered transplant but rather an active cellular component that appropriately responds, and builds upon, this negative feedback cycle via cellular programming.

Could we harvest cartilage from other animals? Such as pigs perhaps.

That's a strategy but there are issues with immune rejection since that's a xenograft. I think there are some products based off of this concept for other areas (skin grafts) after removing all xeno-cells from the tissue, but articular cartilage is more difficult to remove/replace xeno-cells with autologous or allogenic cells.

I think cadavers are the typical source. Since there's no blood vessels in it, tissue rejection isn't as much of a concern.

Growth hormone? It's just not without risks.

Cancer from soft tissues (sarcoma) has a high mortality rate. Even a very low risk of getting it from growth hormones would drastically limit their use. I believe there was a study showing a minute increase in risk of sarcoma in kids getting growth hormone injections. Which is why pediatric endocrilogists are lot more restrictive these days prescribing growth hormones than maybe 7 years ago. At least in my country.

People lose muscle mass as they age as well. That is why the risk of falling (and not being able to get up) is high among the elderly.

One of the major reasons that people lose muscle mass as they age is due to arthritis making it difficult to exercise. If there was a treatment for cartilage wear then elderly people could more easily stave off muscle loss by working out more.

Yes but this is mostly treatable by simply continuing to exercise and perhaps supplementing with creatine as you get older. There is no real way to prevent cartilage wearing down.