The on-disk format can evolve, and new features can be added. It being a breaking change is not a big deal as long as you don't mean to rollback to an earlier

The on-disk format can evolve, and new features can be added. It being a breaking change is not a big deal as long as you don't mean to rollback to an earlier version of ZFS. The problem with this change is not that it's breaking, but that it essentially forks too many code paths in ZFS to be worth doing.