I use my m43 gear mostly for travel, where its small size and weight are big advantages. A small shoulder bag carries the lot of it, and sails easily through airport checkpoints. In use the camera and lenses attract no notice. Real-world image quality differences between a Panasonic GX8 and a Sony A7rii are, for my travel needs and purposes (YMMV), small enough to not matter. Really!
I use my larger format gear mostly with fast lenses shot wide open or nearly so. This is where it has an advantage for me. If for some reason I felt the urge to use 35mm format gear with lenses at ~f/8, after a second thought I'd likely opt for m43 at ~f/4 instead.
-Dave-
Fair enough! Whatever satifies you and suits your purpose is the best option. Most of my photography is done during my travels, so I'm reluctant to compromise the degree of quality that I'm used to, for the sake of a kilogram, or so, less weight. I actually go through the baggage check with my D810 and 14-24 zoom slung around my neck so it's not included in the weight of my carry-on luggage. I've never been asked to weigh the camera that's around my neck.

However, I can see the advantages of a lighter system, obviously. A few years ago I bought the Panasonic Lumix FZ200, which has a very impressive, fixed, 25-600mm/F2.8 lens attached. I wondered if this camera would satisfy me, regards image quality. I spent a lot of time comparing the image quality with equivalent focal lengths from my Canon and Nikon DSLRs, and it became clear that the Panasonic was no match, so I sold it to a neighbour.
There is also another issue which might tend to be glossed over. Whilst the image quality at the long end of an MFT zoom might compare very favourably with the significantly cropped image from a full-frame lens, the purpose of a zoom is to provide a number of different focal lengths to use, without the need to change lenses. The image quality from those wider focal lengths, from the D850 for example, will tend to be significantly better.
Let's take the example of one of BJL's dream lenses, the Panasonic 100-400, which becomes a very impressive 200-800 in full-frame equivalence. The closest equivalent zoom on the D850, to get the same reach, might be the Sigma 150-500, about 800 grams heavier.
A 1.6x crop at 500mm results in a 17.5mp image at 800mm equivalence. The resolution from the 20mp Micro 4/3rds should be better, because of the significant crop of the Sigma lens at 500mm. The difference in pixel count is insignificant, but the difference in DR will still favour the Sigma/D850 image.
The over all impression might be that the Panasonic image at 400mm is at least slightly better. It would be interesting to see real world comparisons.
However, what are the quality differences at wider focal lengths with each zoom? At all focal lengths between 100 and 250mm with the Panasonic lens, the comparison will be between 20mp and 45mp. Downsampling the 45mp image to 20mp, for the sake of a sensible comparison, the image from the D850 becomes significantly better in all respects.
The DR advantage at the lower base ISO becomes 2.5 EV. Even at ISO 200, the DR is a full 2 stops better. At all other ISO's the D850 image is around 1.5 stops better (compared with the Pen-F).
At the pixel level, the greater SNR (at 18%) is only noticeable at the lower ISO of the D850. However, when the image is downsampled to 20mp, that SNR advantage becomes much more significant. At the lower base ISO of the D850, it's about 7.1dB better, and about 5dB better at all other ISOs.
https://www.dxomark.com/Cameras/Compare/Side-by-side/Nikon-D850-versus-Olympus-PEN-F___1177_1070Color sensitivity and tonal range are also noticeably better. I would expect resolution also to be better, even if the Sigma zoom is of lower quality than the Panasonic.