I know all too well how details can prevent otherwise simple solutions. But for the full disk system, a simple solution is to not allocate all the available spa

I know all too well how details can prevent otherwise simple solutions. But for the full disk system, a simple solution is to not allocate all the available space, surely it cannot be hard to reserve a little bit such that you will never encounder the full volume issue. You mention it shortly, and I belive that ZFS was designed for a world where growing existing arrays, reaching 80+% utilization, or using different block sizes all is things that never happen anyway. ZFS could do with a rewrite to allow this, but it would not be ZFS compatible anymore. But I will not deny that would you describe could be a telltale sign of a development team Apple simply cannot work with. But nothing prevent Apple from forking OpenZFS and work on it in house (perhaps apart from patent issues).