Micro 4/3 saved my ability to continue to be a serious photographer. Some years ago, I was hiking in Yosemite with a backpack full of Canon FF gear. My back started killing me and I almost tossed the gear in the trash. I realized that, after multiple back surgeries and getting well into my "senior" years, I just couldn't carry that kind of weight. Plus, I've got an absolute rule that I follow when in the field: I NEVER change glass while hiking. NEVER. The risk of dropping a lens, or introducing dust into the camera is simply too great. For example: On the 1st day of a Nat Geo photo expedition to the Galapagos, one of the guests dropped his main Nikon lens while walking on a rather irregular path. He was then stuck with extreme wide angle, or long tele for the rest of the trip. I never do that unless I find a clean, stable place to swap lenses. Plus, and maybe this just my luck, when shooting wildlife, something would always happen for which I had the absolutely wrong lens on the camera and there was no time to change. I consider the camera body to be an extension of the lens and if I'm carrying 3 lenses that I intend to use while on a hike, I need 3 bodies. Yes, I know that many new FF mirorless bodies are not much heavier than a micro 4/3 body. But, the glass is still MUCH heavier.
When Olympus introduced the OM-D EM-5, years ago, I bought one and ran a bunch of tests vs my 5D mk2. Of course there was a difference in IQ, but it was much less than I expected. Mostly, it required pixel peeping to notice. I had an upcoming trip to Yellowstone and I brought the EM-5, 2 Pen bodies and 3 lenses and NO backpack. I was able to carry all 3 on a BlackRapid strap system and whenever I saw something interesting, I just raised the appropriate body/lens combo to my eye and fired away. Of HUGE importance to me, I was able to hike 5-10 miles at a time without back pain. In reality, for that kind of trip, if it wasn't for the light weight of the micro 4/3 gear, I would probably have just hiked with a Sony RX compact camera and left the Canon in the car. So, for me, the choice isn't whether micro 4/3 can compete with FF, but whether I bring a camera or not. I'm a volunteer photographer for several organizations, including the National Park Service, and have never received a negative response to images I have submitted.
Today, I have an OM-1 and a couple of older E-M1 mk2 bodies. The IQ is far superior to the original micro 4/3 gear and the new noise reduction apps (I use DXO DeepPrim XD) enable me to shoot over ISO 10,000 and still obtain clean results. Would I like a camera with 30-40MP resolution, sure. But, absent someone willing to carry it for me, it simply would not be with me when I'm doing my favorite kinds of photography. So, what good would that do?