You're assuming all this is "really really unhealthy" but what is the justification for that opinion?
And when I go to the park and have a run, of the 100 people I might see there and on the way, we're closer to 50/100 than 5 or 99. So I think we have a problem.
The flaw in the whole argument is that somehow people are having less "meaningful" conversations because they headphones on. I'm sorry but you're not going to have a meaningful conversation (or any at all) with the 100 people who are also actively running at the park whether or not they had headphones on. I still don't see it as a problem if 100% of the people running had headphones on; they are there to run! It's like saying there's a problem because 50 out of 100 people at the park having running shoes on. If you've pre-decided that running shoes are a problem then that's a big concern otherwise it's just nothing.
For me, if I didn't have headphones on I wouldn't be going for a walk/run at all. That one thing has drastically changed how I approach exercise in general and I would do less without them. That said, I do occasionally enjoy a nice walk/run without any headphones but as the exception rather than the rule.