I'd suspected performance might not be the primary purpose when I came across the Python packages for implementing filesystems.
Still, it seems like a lot of the interesting filesystem ideas have to do with non-root users authenticating to something else over some network connection. This type of thing is simply a more natural fit for user space.
Keeping the high-level stateful protocol stuff out of the kernel is usually a good idea except when performance is all that matters (i.e. there will be full-time developers tuning it and cleaning up the inevitable security and crash bugs).
Still, it seems like a lot of the interesting filesystem ideas have to do with non-root users authenticating to something else over some network connection. This type of thing is simply a more natural fit for user space.
Keeping the high-level stateful protocol stuff out of the kernel is usually a good idea except when performance is all that matters (i.e. there will be full-time developers tuning it and cleaning up the inevitable security and crash bugs).