Illumos is hardly kicking it in the mainstream either. I'm just saying it can't really claim to be the last descendant of legit System V.
There is a world of difference between an OS which last major release was well over a decade ago and now only receives patches every 12 months; and an OS that was released several years latter, is still under heavy active development and has several commercial distributions.
Just because something isn't "mainstream" it doesn't mean it's equivalent to a deprecated platform that's only supported on legacy iron.