The reason for the disappointment in µ43 images is because of the small pixel/high density issue. Any slight shortcoming will be magnified more than the FF equivalent. Just as MF prints looked so much better than 35mm prints. At small sizes and excellent technique (aided in no small part by Oly's excellent IBIS) µ43 looks splendid. My 5DsR looks splendid at FF and when cropped in close still looks great. But only with tripod mounting, careful focus and strobe.One of the reasons that birders like APSC is the field of view in the viewfinder. In a 5D/6D body, with a 400-600mm lens attached, a small bird is still very small. It might turn out ok in crop, but it's hard to tell if you're even in focus through the viewfinder with a full frame camera, without going to magnification liveview, even when you have enough pixels to make a decent photograph.
For me, one of the real benefits of a high res mirrorless is that if you can toggle between crop and full frame in the EVF with the push of a button, so you can use the wider field of view to compose -- or just find your subject -- and the cropped field of view in the EVF (and further magnification if you want it) to focus, or to simply end up with much smaller files instead of cropping in post. Plus, you can't shrink the original RAW by cropping.
Obviously, we have no idea how the high resolution Canon sensor will perform. But here's the issue with high-resolution sensors, to date -- they just aren't a replacement for having longer (or ideal) focal length lens, but it's sure tempting to try to do so. A MFT 300mm can be razor sharp, and yet the photos are not as pleasing as a 1DX with a 600mm at a much lower resolution. Or to put it another way, if you deeply crop 3 megapixels out of a 50 megapixel photo, it won't look remotely as good as if you had enough focal length to take 50 megapixels, and then reduced that to 3 megapixels.
Olympus MFT is the system I had after I switched from Nikon and before I came to Canon. One of the problems for it is that there simply are no 400mm-600mm+ lenses, and you can't get to those focal lengths without shrinking the aperture to way too small. In real terms, I don't have an issue with a full sized body and a 100-400 II lens mounted -- I can hike and shoot with that for 10+ hour stretches, and the results I get are just superior than an MFT with a 300mm, especially in bird in flight photos.
The reason I left Oly wasn't that I didn't like their cameras -- it's that I felt like I hit a ceiling with BIFs for even large-sized birds (like heron), where the keeper rate was significantly lower than Nikon, and I was frustratingly unable to increase it. Of course, that was a long time ago, and perhaps things have changed -- but now, I am a very happy Canon shooter, and for my non-birding stuff, Canon just has a much more complete lens portfolio with good options in a spectrum of price points. Plus, the third party accessory ecosystem is second to none.
Putting price aside, at 400mm and above, and especially with mirrorless, there is no advantage to MFT or APSC for the body, since the lens does not get any smaller with a smaller sensor, and I personally find that a larger body acts as a better balance to the heavy glass.
Anyways, all this to say, I'm excited to see how Canon's 100 megapixel RF performs. If it offers the kind of leap that the 5DSR did, I will probably buy one.
So going to 100MP will yield an all-in -one package but that added crop mode may be far less useful than one might hope.