Aren't they all hobbyist operating systems now? Nobody can seriously claim that OS3.x or any of the "next gen" amiga operating systems that followed exist for any purpose other than for our amusement. Not that there's anything wrong with that, of course.
You could look at it that way, but "hobbyist operating system" makes me think more of roll-your-own projects than keeping an older OS up and running.
It's a pity there aren't more hobbyist OSes for non-x86 systems - it'd be fun to see something like
KolibriOS or some other minimalist assembly-based hobby project on the Amiga or other platforms.
