Exactly, BcacheFS, HAMMER2 are other file-systems that are being worked on by some REALLY good programmers for years and years to come. It seems like a file-system takes longer to stabilize than a kernel.
For good reason in my opinion. An unstable kernel will cause application glitches, unnecessary slowness or OS crashes. They are annoying but not persistent (ie a reboot and you can carry on for a bit). However an unstable file system are persistent and thus could destroy all of your data forcing you to recover from backups (assuming you're diligent enough to keep tested backups).
Plus file systems still have to deal with buggy consumer hardware and other similar edge cases (eg storage devices, power failures, etc) just like a kernel would.