I don't know enough about macOS system internals to confidently speak about them, but my guess is that there's some kind of legacy cruft/holdover that's prevent

I don't know enough about macOS system internals to confidently speak about them, but my guess is that there's some kind of legacy cruft/holdover that's preventing a direct disk image replacement for updates. Wouldn't be surprised if this were fixed in the next major release or two, likely paired with dropping support for something.