Maybe, but it would only make sense if that crop mode enables faster shooting. Otherwise I'd be pointless.
I agree--that would have to be part of it. Less data to pull off the sensor should equal a faster frame rate, presumably very close to the M6 II's. So a slow(er) full frame could also be a fairly fast APS-C. (I don't know how current crop modes work. Do they imply a higher fps as things are currently designed, or would this be a new way for Canon engineers to think about those things, because cropping happens in the RAW file, not the electronics?)
The point has been made that this would end up being a $4000 camera, and that doesn't seem like a ridiculous number for what it actually is. On the other hand, if it's basically what I'll call an R6--it could be fairly slow in full frame mode so long as it speeds up for crop mode, and maybe that can make it less expensive.