It wouldn't surprise me if most people using PHIGS/PEX in 1989 were doing visualization for scientific computing on workstations, and any UI or graphics was an afterthought compared to the number crunching code that was generating the data to visualize. If so, they were probably using Fortran, and there's probably lots of people doing that sort of work in Fortran even now.

Not that I would recommend using Fortran for anything, just providing context. Even Numerical Recipes didn't get a C version until 1988. The original edition from 1985 was Fortran and Pascal. So even Pascal was considered a more mainstream language for this sort of work than C, at that time.