Something that doesn't get discussed is something that strongly influenced me in early 2012 to start using m4/3 rather than my DSLRs. It is that the base ISO is 200 for the m4/3 bodies. That means that even with comparatively slow lenses such as the Olympus 14-150mm f4-5.6 (sort of equivalent to FF 28-300mm f8-11) that much of the time even with that rather slow zoom I would still be able to shoot at the base ISO.
I have all my photos in Lightroom so I just checked. Looking at all my Olympus files (PEN-F, E-M10II, E-M10, E-M5) taken in all those years I see that about 83% were taken at ISO 200. And 89% were taken at <= ISO 400. And the Olympus bodies also have great IBIS. Of course, one can move up to ISO 800, 1600, 3200, 6400, etc. and I have at times, but I rarely need to. When the light really gets low I generally put on a lens such as the Panasonic 20mm f1.7, Olympus 25mm f1.8, Olympus 45mm f1.8, or even the Panasonic 14mm f2.5.
In all my years using film in the 1970s, 1980s, 1990s, and until 2002 when I was shooting color slide film I was using ASA 25, 50, 64, and 100. With color print film I was using ASA 25, 80, 100, 200, and 400. With B&W print film I was using ASA 125 and 400. For many international trips I used ASA 100 film. For example, in 1995 I spent a few weeks trekking in the central highlands of West Papua New Guinea -- Irian Jaya and I used a Minolta Alpha 707si SLR + Tamron 28-200mm f3.8-5.6 and a Minolta 50mm f1.7. Almost all the photos except a few at night were taken with the slow Tamron. I only used Fujichrome 100 ASA 100 slide film. No IS either. Oh boy, if I had had an m4/3 body and a couple of lenses I could have done much better!
The rather fast base ISO and IS really helps with regards to using somewhat slow lenses. Of course, with APS-C and especially FF this helps a lot too. With FF f5.6 and f8 lenses along with great high ISO and usually IS means that for most people the fast f2.8 and faster lenses just don't make a lot of sense. Smaller, lighter, less expensive FF f5.6 and f8 lenses are quite usable for the vast majority of people.
The info I can get from Lightroom is cool.

I did some more checking and I see that using my 4 zooms (all with modest apertures f4-5.6, I don't have any f2.8 zooms) in the last 8.5 years my photos with these zooms were:
82% <= ISO 200
90% <= ISO 400
95% <= ISO 800
97% <= ISO 1600
99% <= ISO 3200
Also, I found that even with these modest aperture zooms I shot 71% at f4-5.6 and 29% at > f5.6 (stopped further down).
This is just my experience shooting what I tend to shoot, but the modest apertures along with the m4/3 low ISO setting of 200 (and IBIS) means that I am rarely forced to go to ISO above 400.
All that tells me that for the way I mostly use my cameras a FF body with these new slow zooms would be just fine. Most of the time even with the modest apertures I would still be shooting ISO 100-400, but no worries at all about going on up to 6400 or even higher on FF.