You have to control for all the people who don't do an activity because they're too unhealthy to continue. And there is no obvious way to do that.
1. Seems like they identify someone as runner or walker based on a questionnaire filled. After X years those people fill another questionnaire to indicate whether they had OA or hip replacement. I would think that at the time of the second questionnaire a runner will still be assumed a runner, even if he doesn't run anymore.
2. Exercise other than running increased rates of OA and hip replacement.
Of course, I'm being facetious. Such a study would never be done because it's completely unethical. But the point is that you can't determine an intervention (running) is safe and effective without a proper trial like that. A survey that asks people whether they're a runner or not, before even beginning, has a problem with selection bias.
Unfortunately they didn't break down the km's ran to classify the individuals. Also The runners group was almost more likely to do 'vigorous exercise'.
To answer your question, they did continue to classify as runners even if they stopped (and almost half did stop).
The problem is not that you're being facetious, the problem is that if you insist on a single study proving something with a 100% certainty, we shouldn't be doing any science. For example, I can find numerous holes in your supposedly perfect study design. It turns out that active runners have less joint issues indeed? How can you be sure that it's due to running and not due to increased exposure to sunlight? Why aren't you testing different exercises? What if the benefits of running are solely due to improved cardiovascular performance but running specifically is actually bad on the joints bearing the heaviest load? If you dig deep enough, we can go on and on.
The burden of proof is on running advocates to show that running is safe and to clearly lay out the conditions for safety. As a former runner (who quit after breaking an ankle and speaking to an orthopedic surgeon who saw runners all the time), my strong suspicion is that running is only safe for those who:
a) run with perfect form
b) have perfect-fitting shoes
c) eat a well-balanced athletic diet
d) are already slim and fit before they begin running
I feel like that last one is the real key. If you're massively overweight and someone tells you to just go for a run, you're going to get hurt. To be a runner you need to get in shape first and have someone help you buy proper shoes and teach you proper form. Anything other than that is bad advice.
Massively overweight people can't run, even if they wanted to. Their "run" speed is slower than my "walk" speed.
I'm mystified at why you'd say he was trying to "claim that running is perfectly safe" when his first sentence was, "Like all repetitive activities, runners are prone to overuse injuries and the most common is to the knee." Maybe you should go back and get a grip on precisely what point he was trying to make. It was _not_ that running is perfectly safe.
People who run are prone to overuse injuries, no doubt about it. People who are long-term runners tend to learn how to manage injuries and keep them minor. Some people never learn.
I've discussed with running friends that in a way it seems odd that medical insurance covers treatment for our running injuries, when they are injuries that are in a sense "chosen"; we could certainly have avoided them if we didn't run at all. Yet, still, a runner's visits to physical therapist, orthopedist, or sports medicine doctor are covered. The answer we come up with, and it seems correct to me, is that covering runners for their "chosen", "elective" injuries is over the long term still much cheaper than covering non-runners for the much more expensive health problems they will have over the course of their life.