Also to this, I think much was lost, when the Andrews X system lost
impetus. The filesystem lived on (they always do) but somehow, people
lost interest in WYSIWYG built around the X fonts, and Troff. I rather
liked it, used it heavily. I produced the university Phonebook for a
while in it, and adjunct scripts. (EQN for some things)
There was a rather nice animation tool, somebody obviously either was,
or was dating a ballet major at the uni, and decided to code something
pretty: the worked example was very dance-oriented.
(I know this is a layer "up" from X itself, but it goes to what might
have been, if the critical mass had been there)