FUSE effectively provides a POSIX-like interface to arbitrary code, whereas the author is lamenting that these features weren't built into Unix/POSIX-like systems to make them widely available in the first place.
The "problem" with modern POSIXish systems is that the definition of what is "POSIX" seems to be set in stone. All the various Unix-likes offer POSIX compatibility (or aim toward it) while new features that extend capability end up being implemented in completely different ways across different systems.
So for example while Linux has inotify (arguably an implementation of the filesystem traps that Multics had), SGI had FAM, FreeBSD has kqueue, MacOS has FSEvents, and they're all incompatible with each other... a nightmare for developers of portable applications.
The "problem" with modern POSIXish systems is that the definition of what is "POSIX" seems to be set in stone. All the various Unix-likes offer POSIX compatibility (or aim toward it) while new features that extend capability end up being implemented in completely different ways across different systems.
So for example while Linux has inotify (arguably an implementation of the filesystem traps that Multics had), SGI had FAM, FreeBSD has kqueue, MacOS has FSEvents, and they're all incompatible with each other... a nightmare for developers of portable applications.