Right. So hard? Its a lot of work that is unplesant and nobody wants to do, I think that qualifies as "hard". I do agree its impractical, but it seems that for something as important as a function, working file system (and don't suggest that butterface fs is a production ready alternative) that somebody would put in the work to make it happen. After all the file system is everything, if you can't safely store your data, whats the point?
As an addition to the above, clearly ZFS is better than the common alternative: raid cards running some unknown firmware (that is most definitely open).
