20 or so years ago, I was trying to carry the weight but now just don't want to or can't.
As for hand holding a XF and 150 or 100MP back, it's up to an individual's style, what they are shooting, and where they had to go to get it.
2017, my first trip back to California in 20 or so years, I had both the GFX and XF/3100. Goal was to shoot the XF/3100, no doubt it takes great images. 2nd day out, I realized, just wasn't meant to be unless I was working from my car. 3 thousand to 6 thousand feet, out of breath at most stops, no way was I going to attempt hand holding the 3100. Thus I had to break out the tripod, setup and take the shot. After about 20 to 25 of these stops, for me it just wasn't fun anymore, used to be, just is not. For the rest of the trip on hikes, it was GFX. With the massive flow of water, never needed to even attempt slower shutter speeds.
But take an XF, with a 40-80 or the 75-150 and 3100, you are taking close to 7 maybe 8 pounds. Even the 35mm Blue Ring is a 5 to 6 pound hold. I can hand hold with the 45mm like Doug did or the 55mm, and of course the 80mm. But trying to even think about the 240mm, just can't do it. The "vibration" system on the XF, if set to infinity, won't fire when you hand hold at least mine won't as I never reach a position still enough for it to register. B of these lenses are top end at least mine are, and are capable of capturing excellent sharp images, but not hand held, not very often.
I have taken plenty of shots with the GFX and 250mm and the 1.4 converter, all or most hand held and there the difference is OIS. It's no different in the 35mm world, with all large glass you have VR or IS, depending on the company. And it does make a difference. There is still a lot to be determined on the 100s Fuji, as for now, it has to be one of the most ungainly looking cameras I have ever seen not what I was expecting as a follow on to the GFX.
The E shutter on the 3100 is a huge asset, but you can't hand hold it, period. You can with the D850 or GFX, not that it's a great idea to do it, but there are times it may be helpful. I have never been able to even hand hold the 3100 with Live View to try and get critical focus on a scene, pretty much impossible due to movement. I do it all the time with both the D810 and D850. You can't really do it (at least I can't) with the GFX due to the way Fuji has implemented the 100% view of Live View.
Sure it can be done, and is done but I still feel the best way to get the most out that 100MP back is on a tripod.
Paul C