[CWuestefeld]

Not sure why we need 6400+ today, except for pissing contents and/or stalking).

I agree with your overall point - and I've pretty much abandoned my APS-C camera, using my M43 camera almost exclusively.

However, there is a good reason for better high-ISO performance. It allows for tradeoffs in other areas. It means we can hand-hold in darker lighting or at higher focal lengths or smaller lighting than anyone could before. The ability to take a sharp photo while out on a hike when I didn't want to lug a tripod is valuable.