I suspect that the useful lifetime of running shoes depends upon the runner's weight. As a heavier runner/walker trying to control my weight, I found the 500 mi/pair to be reasonable when running.

I could walk longer in them w/o problems. I'm currently rehabbing a strained hip and walking 35-40 mi/week and need to replace shoes about every 750 mi. I suspect a lightweight runner could get a longer useful life from the same shoe. Like many others, I have found the uneven wear patterns on my shoes reflect issues with my foot strike that I have improved but not eliminated.