I'm an APS-C shooter. That's my choice. I shoot mostly with teles and rarely wide angle. Even if you were to convince me that 17MP really was enough for the shooting I do, if I got an R5, I would have to manage raw files that are 2.5x as big as I need, just so I can crop away 60% of the image area. That doesn't make sense to me.
Cogent points all, except that I believe that in crop mode the R5 really does remove the data in camera so you don't have to deal with the extra large files. (If I am wrong I am willing to stand corrected.) You can choose to go into that mode (with a full frame lens) or will be put in it automatically (with an EF-S lens). However I know from personal experience that the automatic switchover does not work with Tamron lenses! You end up with a full frame file with most of an image circle shown, like you were looking through a pipe. (The top and bottom of the vignetted circle are clipped off, of course.)
It's possible that instead of getting an APS-C camera you'll end up being told to use an R5s with the same sort of crop mode, which would at least give you in the neighborhood of 30MP cropped. That, to be sure, would be massively expensive. One would hope whatever it is they end up offering (if they offer anything) gives you at least that much.