> I shouldn't need to hackjob my kernel to make a single individual lone drive work.
You don't. You only need the hack if you do a bad job of cleaning up after the loss of the other half of your two-drive mirror. ZFS won't let you transition in-place from RAID to non-RAID at all. Btrfs just requires that you not reboot in the middle of that migration.
Seems insane, after all not all reboots are voluntary.
"Insane" is pretty strong for a temporary limitation that is just as severe with traditional RAID arrays. A sudden power failure or any other hardware problem cropping up during an array rebuild is a nightmare scenario.
It should be noted that md-raid does handle that scenario. I agree with you that the characterisation of this present limitation in btrfs is quite unfair, but not all RAID systems are susceptible to that problem.
And yet it hasn't been fixed despite being well-known? That tells me to stay away from it more than anything else.
"even if you do trigger it a one-line kernel patch will bypass the overzealous safety check"
I shouldn't need to hackjob my kernel to make a single individual lone drive work. If you can't work with a single hard drive, you have no business trying to work with multiple hard drives.