As efficiencies in cutting-edge hardware improved, the gap closed. Intel and AMD leapfrogged the smaller design firms running these companies, and more and more vendors threw in the towel on hardware design, switching over to standard x86 hardware. By the early 2000s, distinctive OSes like Solaris and NEXTSTEP were just legacy GUIs that could be installed on commodity PCs, although many flavors were discontinued outright in favor of Linux, leaving these companies (several of which were swallowed by HP) without any moat or vendor lock-in. (Notably it happened to NEXTSTEP twice, once in 1995 and again a decade later when Mac OS X 10.4 was officially released for Intel CPUs.)
Every Microsoft developer had a Xenix workstation for things like email, access to network disks, running a decent C compiler, and debugging.
DOS was practically a single-program environment with no memory protection and no networking. Unix offered much better productivity for software developers.
Engineering in general was a field that used Unix workstations heavily. Microsoft didn’t become competitive until Windows NT in 1993.
Linux would never taken off in such alternative realities.
Not that it matters that much now with WSL, and Azure Linux.
Same as we use it now, to be frank. Unix workstations as an interaction model have persisted so long because it works just great.
I was writing a lot of Unix software in that period - database apps, business logic, and so on. For me, using an MSDOS-based system was a compromise, which I enhanced by using Desqview to get multi-tasking - it allowed multiple MSDOS instances on a single machine, in which I ran terminal software, compilers (our apps were being ported to MSDOS...), and database admin tasks - just like today.
What we have today in the form of MacOS or Linux workstations is pretty much what we had back then, too. The power is inescapable.