But presumably if said changes preventing DKMS usage were reasonable they would have been merged anyway independent of bcachefs, and likely with less drama and disruption? I'm not suggesting that there aren't some silver linings to the cloud, but it doesn't seem like the result is anywhere near neutral (let alone positive) for anyone involved.
Yes and no - the kernel interfaces only reflect what the kernel itself needs. It doesn't to my knowledge maintain interfaces for the purpose of enabling out-of-tree modules.
Changes would therefore need to be an improvement for in-tree drivers, and not merely something for an out-of-tree driver.
