[DanielDent]

You can send/receive to the same zpool and still defrag. With careful thought, this can be done incrementally and with very minimal availability implications.

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.