As a birder (not only but I love doing it, did it yesterday again) I'd add that birding is a trade-off between pixel size and the light-motion problem. With the latter I mean that birds are often moving fast, smaller pixels require higher shutter speeds for freezing them sharply on the pixel level, but the amount of light available is finite. This is why I sometimes prefer to use my old 22 MP only FF 5D3 over my crop sensor 7D2, when I shoot birds. With less light, the 5D3's images tend to be much more "crisp" on the pixel level. So then I really prefer such a bird image over the 7D2's mushy output with more pixels in the bird's image area but less information on the pixel level. Of course, I know I could downsize an image to get about the same results (if the camera's sensor technologies are about comparable), but I love get the out-of-camera images as good as possible w/o too much post-processing. (Maybe I am sort of old-school-corrupted because I still shoot film in parallel to digital.)
CURSE THOSE LAWS OF PHYSICS!
I hit that same quandary a lot. The crop camera has a greater pixel density so I get more pixels on the target, but the FF camera puts better quality pixels on the target. If the light is great, I tend to go crop, and when it gets a bit dimmer (or I am not zoomed to the Max) I prefer to go FF. My crop camera is faster, but the FF camera seems to have more accurate AF. The crop has a far better spread of AF points.
And my phone beats them both for convenience......
And my P/S is waterproof with terrible ergonomics and IQ ....... but it is waterproof!
I find each has its place. No way will I say that one is better than the other for everything, it’s sort of a “right tool for the job” type of thing. We here a lot of comments on this forum about why can’t Canon make a camera that meets MY specific needs, but so often those needs are conflicting.
If Canon does come out with a high megapixel R, there will be a lot of cursing and wailing on this forum because “that isn’t the camera I want”, but for those it is targeted at it will be a great camera. For those who it isn’t for, be patient, your turn will come.