I think the mechanical shutter in the R3 is there purely for the more conservative users that aren’t trusting of the electronic shutter. Frankly not having a mechanical shutter is making me a wee bit nervous with the Z9 and that all the RAW is compressed. But in saying that, I am sure I’ll get better images from it than from my Z6 or at least as good and with so many shots over ISO 1000 to get high shutter speeds it’ll be irrelevant if it is 12bit or 14bit or 14bit lossless compressed for me. If I want more IQ I can shoot at ISO 64.
Yes, undoubtedly there will be some users who don’t want to be solely reliant on an electronic shutter, and that may have influenced Canon’s approach to the R3.
I’m sure there are some scenarios (e.g. fill in flash in bright sunlight) where even a very fast readout electronic shutter can’t sync fast enough, and there may be other scenarios that don’t immediately spring to mind.
My R5 doesn’t allow exposure bracketing with electronic shutter, it has a fixed (maximum) burst speed of 20fps, and it doesn’t have any means of producing a volume-controllable fake shutter sound. If those issues didn’t exist, I’d be absolutely happy to have an electronic-only shutter.