I doubt relevant people have it this black and white. Either they decide only based on size&weight or they don't care about it at all.
It is kind of a dumb way how to choose a camera.
Not for everyone or every context. I still have my original M and nothing beats the M+22mm in terms of being a camera you can grab and keep with you without really thinking about the fact you're carrying a camera. Casual shooting scenario.
Size/weight also become an issue for extended hikes. If I was trying to hike the PCT I would get a M6 II and 11-22 IS specifically for that hike and that would likely be it.
But for a primary camera I would choose the R6 over the R8 just for IS and mechanical shutter speed. And I'll still grab the 5Ds for stills every time unless I specifically need something the R6 has (IBIS; ultra high ISO; 12/20 fps).
I guess that supports your statement that size/weight are either everything or nothing. But I can't agree they're "dumb" criteria. When they're everything...they are everything.