This isn't exactly what you're asking about (I think you're challenging swimming as a go-to activity for someone too injured or otherwise unable to run) -- but there is a related myth, which is that running ruins your knees. Studies have shown evidence the opposite is true: that running strengthens and thickens bone. I mention this by way of saying that, with the amount of myth and half-truth out there as relates to running and the impact on the body, I'm never surprised by what people believe anymore.
I was always under the understanding that running (potentially) ruins your knees by damaging your cartilage - bone thickness or density isn't the issue. That's why swimming is often recommended because there is no impact on your cartilage.
I'm running cartilage mapping MRI sequences on knees today - and while its hardly scientific, I would have to agree. The big runners I scan have some pretty munted hips and knees. So very messed up that I can't find any decent cartilage to use as baseline normal for the scanner.
This is precisely why I'm a cyclist and not a runner. I like to think that I gave running a sincere effort, but the little aches and pains just bothered me constantly. Maybe I had poor form, or maybe my body just isn't built for it, I don't know. By contrast, after tens of thousands of kms on the bike, all I ever get are sore muscles after a hard effort.
"A study at Boston University School of Medicine looked at the continuous impact of the foot with the ground and the commonly accepted belief that running causes degeneration of the knee and can lead to all sorts of arthritic conditions.
Said lead researcher and epidemiologist David Felson of BU: “We know from many long-term studies that running doesn’t appear to cause much damage to the knees. When we look at people with knee arthritis, we don’t find much of a previous history of running, and when we look at runners and follow them over time, we don’t find that their risk of developing osteoarthritis is any more than expected.” Felson added that recreational running doesn't increase the risk of arthritis."
I recall that the research I've seen on this in the past showed reduced bone density etc. only in serious endurance runners (e.g. multiple marathons, ultra-marathon) and indicated improvements in your "average" runner.
Bone density is definitely an issue, especially for women.
Running is vary dependent on age, stride, body size and style. For example a tall and slightly over weight runner with a poor stride has far more impact stress than shorter, thinner, runners with a more efficient stride. Add to that the importance of not running every day to tive the body time to heal and you end up with wide range of issues.
