I think this was less important then, than people sometimes think.
I recall those days. In the 1980s and 90s I worked as a scientific programmer in a university department. Some of our software was commercialized and sold and supported as a product for a time in the 80s. Pardon the following long memoir, but I think some reporting on what actually happened then, as seen by even one participant, is pertinent.
We used a VAX with DEC's VMS operating system. Our application was developed in DEC Pascal (which didn't have the limitations of Standard Pascal because it used the DEC CLR, Common Language Runtime). Later on we began using Allegro Common Lisp for some things.
Through the 80s and early 90s, we never used Unix and C. And, we were not unusual, even in a university. Most of the VAXes at that university ran VMS (or one of the DEC-10/20 OS in the early 80s), including the computer science department (which began running Unix on some but not all systems later in the 80s). So Unix was not as pervasive in the 80s as some people seem to think.
About "free beer": running Unix on a VAX in the 1980s was definitely not "free", it was a major investment in time, effort, and yes, money (in the form of salaries). First, the OS wasn't a separate line item. You bought a bundled system including both the VAX hardware and the VMS OS. Then the DEC guy came and turned it on and it just worked. I don't even know how buying a bare VAX and installing your own OS worked. How did you handle DEC field service? They required their own utilities that ran on VMS. If you used Unix, you needed an expert in Unix to install it and maintain it.
And it was no different with the early commercial Unixes. You bought a Sun workstation and it came with their Unix bundled (Solaris or whatever). In the 1990s we switched from VAX/VMS to HP workstations that bundled HP-UX, their Unix. In all of these Unix platforms, Unix was bundled and you did pay for it, it was just included in the price.
I think there is some confusion about the history. The free, frictionless, install-it-and-run-it-yourself OS was not Unix in the 80s, it was Linux in the 1990s. By then C and Unix-like operating systems were well established.
Also, there was genuine admiration for Unix technical features, notably its simplicity and uniformity, even at sites like ours that didn't use it. There were several projects to give VMS a Unix-like userspace. There was a (yes) free Software Tools project (that was its name), and a commercial product called Eunice. People who had already paid for VMS paid more for Enunice to make VMS look like Unix.
Unix was a better platform for teaching CS than VMS or the other alternatives.
VMS did come with source code. It came on a huge stack of fiche cards, along with several pallet-loads of hardcopy documentation in binders.
There was nothing like the books The C Programming Language by K&R, or The Unix Programming Environment by Kernighan and Pike. Or the many Unix and C books that followed them. And then the college courses that used them.
Instead there were special courses in system programming and OS internals (separate courses) from DEC. The university would pay for them once in a while. A DEC expert would come for a week and programmers from all the VAX sites would get together all day every day in a classroom while they lectured. There was no textbook, but everyone got a huge binder of printed notes.
So systems programming on VMS, and I suppose other non-Unix platforms, remained an esoteric, inaccessible art, totally divorced from application programming, that used a programming language that was not used for anything else.
A few words comparing my experience programming in C in the 1990s to programming in DEC Pascal in the 80s: C wasn't much worse. The greater safety of Pascal did not make much difference in application programming. In Pascal, array-bounds errors etc. produced a crash with a traceback. In C similar errors produced a crash with a cryptic message like "segfault". But often the actual defect was far from the line that crashed, that appeared in the traceback, so the investigation and debugging was similar in both languages. But the more common (and often more difficult) errors that just computed the wrong answer were about the same in both languages.