> I'd assert that most of the adult population in first-world countries are far more at risk from a lack of sufficient physical activity than from overtraining.

You're using the wrong reference group here. Would you still assert that most people when narrowed down to those who are running every day are more at risk from a lack of sufficient physical activity than from overtraining? Because that's the group we are discussing – those either doing it or considering doing it.

Most of the adult population in first-world countries are not considering running every day.

As a former runner (competitive trackin/xcountry) and a current recreational runner - I cannot imagine never taking a day off is actually safe for your knees and joints. At least for me, I simply cannot run every single day without developing some sort of tendinitis or other mild pain/ache.

Yep, I don't disagree with your point here at all. I'm a former distance runner who gave himself knee tendonitis by overdoing it, so I definitely accept the risk of overtraining. I guess that I was trying to make the point that almost nobody is at risk of that.