All I'm saying about using ISO 400 as a standard is this: It works for me ... and my equipment. I know that some cameras are lousy at that setting, and some others are perfectly acceptable at much higher settings.
For what it's worth ... I settled on 400 because it makes my life simpler ... and it works for me. In part, it's a throwback to the old film days when I shot primarily Tri-X (400 ISO) or (for color) Fujichrome 200(exposed at 400 because I always did my own processing).
So, I'm just a 400 ISO kinda guy.