It still has unparalleled small size. The a7C is a nice alternative but it's not 42 megapixels. And there are not many compact 35mm lenses with good image quality, and the compact ones tend to be f/2.8 rather than f/2.

True, but the inherent costs of a medium format sensor (not even taking into account the likely extra costs of a curved sensor) mean that the bare minimum you could sell this fixed-lens camera for would be several thousand dollars. I just don't see a lot of photographers being willing to drop that much money on a camera that they can never expand the capabilities of.

Quite a lot of medium format cameras were fixed lens. Pretty much all the TLR’s (except some Mamiya’s). All the folders (to a first approximation), the Brownies, and even today all those Holgas.

A GS645w isn’t much larger than a modern 35mm digital, and is certainly lighter.