This is the margin for visibly better resolution than the various 20-24MP bodies deliver. I was looking for a step up in resolution from what I had, in terms of landscape shooting. In short, I missed the D800's IQ for landscape work. High-Res mode on the m43 stuff addressed this, but was more situational than I was really comfortable with.
2. 20MP or better in APS-C crop. Yes, I want to use crop mode, for 3 things:
i. Cheap and fun manual APS-C lenses. I missed this from my Fuji days. They're cheap to buy and lots of fun to shoot with, plus they're small.
ii. Action. I prefer composing with the full viewfinder rather than cropping in post when shooting any sort of action. Crop mode lets me do that on mirrorless as the display is full-sized unlike on DSLR's.
iii. Turning 1 prime into 2. Since I can get away with 20+MP APS-C IQ for my around-town shooting unless it's really dark, I can use crop mode to lighten my kit. For example a 24mm lens can double as a 35mm equivalent by flipping to crop mode. Same way a 55mm can double as a 85mm equivalent, and a 135mm can double as a 200mm. So 3 lenses in the bag cover 6 focal lengths. Alternatively I could go 15/24, 35/50 and 90/135 for a wider 3 lens, 6 fl set. This also works to turn my wee 28-60 into a 28-90 equivalent, a much more flexible range for walkabout shooting.
The Crop requirement is one big reason I was looking at the Z7 and A7RIV (the A7RIII doesn't quite meet my 20MP or better requirement for crop, Z7 hits just over 19MP and is on the cusp) . Unfortunately Nikon has badly crippled the Z7 in terms of crop modes vs the D8x0 bodies, especially in the case of forcing DX Crop when using a DX lens. The A7RIV lacks the nice 1.2x crop mode, but can defeat auto-APS-C crop which is a big win (a lot of APS-C lenses cover more than APS-C, sometime even being quite usable on FF as is (both the Nikon 35DX and the Sony DT35 SAM are somewhat usable on FF).
My experience with 24MP and lower bodies is that for my uses, APS-C crop mode wasn't really workable for anything other than street work. The A7IV and A7RIII would be mostly usable, but not quite where I wanted to be, much like the E-M5II was in terms of resolution (in fact they'd basically straddle the 16MP E-M5II with 15 or 18 megapixels)
The good news is that I'm going to be very hard-pressed to consider any camera within my price range as a real-world upgrade over the A7RIV for the foreseeable future. It's by far the most capable camera I've owned, and even at 3 years old it's still one of the most capable bodies on the market. Yeah, there will be a A7RV later this year most likely, and I expect a Z7III in the fall as well, but it will be years before those hit my price range. Note my outlay on the A7RIV was the same as I paid for the Z5 this time last year, after trade-ins.