If ZFS wanted to live in userspace (and get Linux's userspace API guarantees) then they could have made a FUSE filesystem instead.
SPL now handles the guarantee to separate ZFS's userland components from Linux's constant internal kernelspace API churn. ZFS, internally, doesn't really know what a Linux is beyond a few little bits here and there.
These changes were made years later, but the developer of ZFS-FUSE disappeared, so they were never used.
The FUSE v3 API further builds on that, and today, it's definitely possible to build a high-performance filesystem using FUSE. In fact, there are many examples of this in the wild: GlusterFS, Ceph, NTFS-3G, and so on.
Today, there's absolutely _no_ reason ZFS would not sensibly work as high performance filesystem entirely from userspace. In fact, it still remains an open issue and a desirable target for ZoL: https://github.com/zfsonlinux/zfs/issues/8
I'll admit, I'm all talk and if it really mattered, pull requests are probably welcome.
End-user dont'care about splitting hairs. Iwas using Linux in the mid-90's and that kind of stance is what turned me off. The sound card would brek, the networking would break, configuring the modem would be hell after each upgrade.
The attitude that you must not break LibreOffice is not okey but breaking the file-system okay 'because it's a driver!' is just untenable.
Making people's computer fail will turn users away. It's not just a Linux thing. When Windows breaks stuff, all hell break loose on the Internet. The problem with OSS, is that its maintainers have no direct, immediate monetary incentive to amend. The long-term loss of confidence is hard to measure, but it's real.