Doing so requires that you have enough zfs filesystems in your pool (or enough independent pools) that you have the free space to temporarily have two copies of the filesystem.
Yes, and that is why I did not mention recreating the pool as a solution. If your pool is big enough or expensive enough, that's still "fatal".
(2) If your pool is big enough/expensive enough, surely you've also budgeted for backups.
(2) This has nothing to do with backups or data security in any way - it's about data availability (given a specific performance requirement).
You're not going to restore your backups to an unusable pool - you're going to build or buy a new pool and that's not something people expect to have to do just because they hit 90% and churned on it for a while.
I agree it's not ideal to have filesystems do this, but it also simplifies a lot of engineering. And I think direct user exposure to a filesystem with a POSIX-like interface is a paradigm mostly on the way out anyway, meaning it's increasingly feasible to design systems to not exceed a safe utilization threshold.