I'm with you there. As long as preload is within a reasonable range it doesn't make a difference. The best evidence is that fretting is almost exclusively limited to the lower stack. If preload were a factor, especially excess preload, we'd expect to see more fretting in the upper.

IME - what's happening is a combination of classic fretting, plus the effects of front end weight and vibration hitting the lower stack harder because the upper only has the preload and not the other factors. I also sense that I see faster failures with loose headset, but that's only a gut feeling.