Backward compatibility in fuse is guaranteed on multiple levels:
- the raw kernel interface
- the library ABI
- the library API
While new major versions of the library might come out in the future, the old versions will
still be usable. Just like gtk-1.2 libraries are still around in distributions, many years
after gtk-2.0 has been released.
I agree though, that in case of drivers where a userspace soultion is not available, keeping
support for old hardware in the kernel is a good thing.