You say that, but jwm was born on Irix I think and plenty of people used that instead of MWM/FVWM and similar. In the end, you just managed windows, so using XFM/XFE with JWM was more than enough.

Also, lots of *BSD people liked IceWM with Metal themes for similar reasons. The Windows UI was already peaking at usability, so they used that among virtual desktops and the power of Unix utilities on shells, tools and services.