Hell, Linux didn't exist at all till '91.
I think Xenix was more just a casualty of the Unix Wars. The victors there
were SunOS/Solaris, AIX, and HP-UX. There were a bunch more walking
wounded that never really achieved much market share.
'In the mid-to-late
1980s, XENIX was the most common UNIX variant, measured according to the number of
machines on which it was installed.'
https://en.wikipedia.org/wiki/Xenix