Solaris AND CDE, ofc Solaris bundled CDE.
And Linux used FVWM + RXVT because it was a much faster alternative
for common Unix work (cli and seldom graphical apps).
Most people used X to run terminals and Netscape, and nothing else. And maybe Gnuplot and XV.
Also, you forgot TKinter for prototyping. Widely used.
Well, on hardly usable... Netscape and a lot of software was statically linked and built against Motif, so most people
ran commercial software in mid-late 90's well.
A good FVWM+RXVT setup was as good as CDE, if not better because the CPU and RAM usage was far lower as I stated.
I remember software like GV, mpg123, ImageMagick... far from unusable.
twm was also usable, depending on the point of view.
And being usable by whoever didn't want to put the money into real UNIXes, didn't mean that Xlib + Athena Widgets were something that one wishes to code for.
I surely didn't, and yes I used plenty of Linux systems, starting with Slackware 2.0.
Well, it was the 90's and even W95 was far more usable, Unixen were designed as
to have a custom DE with highly customized settings even if CDE and Motif
were the standard.
On TWM, FVWM was much faster and lightweight. RXVT flied against XTerm and dtterm. On high CPU loads TWM could be
seen redrawing windows, something atrocious (ctwm fixed that a lot), and,
well, as an example, you could code the core of your software in C and glue it to TCL/TK with relative ease. You had bindings and it was fast enough, much more than the pure TCL/TK.
On usability, most Unix folks would run some XTerm and a biggie application such as one for biology, CAD or astronomy.
If something required more windows, FVWM was perfect to switch between virtual pages. Better than a taskbar.
Most people used X to run terminals and Netscape, and nothing else. And maybe Gnuplot and XV.
On TK, ofc, but calling C from TCL was granted.