I pointed out on the other thread discussing the new lenses, but I'll repeat myself here:
I like birding. Sometimes, in very dark forests, even at high noon, I'm needing ISO 3200, even 6400, and on rare occasions, 12,800 to get the shot with my 5D4 and the Siggy 150-600 (f/6.3 at 600).
Simple math: if a shot requires, say, 1/1000 at f/5.6 at ISO 3200, it will require ISO 6400 at f/8, and ISO 12,800 at f/11. Unless the new cameras can produce the noise and DR characteristics at 12,800 of my 5D4 at 3200, the f/11 is a non-starter for me. As it is, the Sigma is pushing the limits, a bit of a compromise lens in terms of size/aperture/focal length/price. If it weren't a fixed focal length, and a bit heavy, I'd be toting my 500/4 with me very often.