Nonsense. It seems incredibly high to me.

Think about it. Do you know anyone, anyone at all, whom you didn't meet through FT or a similar air travel-fanatic venue, who has ever done even one mileage run?

I certainly don't. Far, far more common are people who groan and grouse about flying and all its indignities.

And on top of that, in many mileage run trip reports in which someone flies into Airport X and then departs on the next flight, the report includes some description of a gate agent or FA expressing surprise that the passenger is leaving after having just arrived. If mileage running were common, airline employees would be completely unsurprised by this behavior.

So I really doubt that there are more than a handful of mileage runs being flown on any given day. If you think there are more, you are succumbing to sampling bias arising from spending too much time on FT.