My take away from trying to follow the discussion on the kernel mailing list was that the Bcachefs developer want to work in a certain way, that Linus does not think that fit in with the rest of the kernel (to put it mildly). Having Bcachefs in the kernel certainly helps with adoption, but I can't help thinking that a kernel module might be more inline with the development process that Bcachefs wants.

The underlying problem might have been importing Bcachefs into the mainline kernel to early in it's life cycle.