In 1979, Brian Kernighan modified troff so that it could drive phototypesetters other than the Graphic Systems CAT. His paper describing that work ("A Typesetter-independent troff," AT&T CSTR #97) explains troff's durability. After discussing the program's "obvious deficiencies -- a rebarbative input syntax, mysterious and undocumented properties in some areas, and a voracious appetite for computer resources" and noting the ugliness and extreme hairiness of the code and internals, Kernighan concludes:
None of these remarks should be taken as denigrating Ossanna's
accomplishment with TROFF. It has proven a remarkably robust
tool, taking unbelievable abuse from a variety of preprocessors
and being forced into uses that were never conceived of in the
original design, all with considerable grace under fire.
The success of TeX and desktop publishing systems have
reduced `troff''s relative importance, but this tribute
perfectly captures the strengths that secured `troff' a place
in hacker folklore; indeed, it could be taken more generally as an
indication of those qualities of good programs that, in the long
run, hackers most admire.