That's an interesting experiment, Dan. And I agree with Dave that 3.5 MPH is not a worst-case scenario.
I think some people tend to make a virtue out of necessity. Since they can't achieve a shutter speed of better than 1/500, why, 1/500 must be just fine! As you point out, making poster-size prints exposes the limitations of even the best modern cameras under gym available-light conditions. Those who don't target that kind of print can afford to accept lesser results.