Didn’t Microsoft somehow ruin Dr DOS? Not technically, but didn’t they sue them or something? Which would mean this is the same issue, 40 years later. Yes, I was there on the 80s, but I had a Commodore 64. We did use GEOS, if that counts. I was not present for the 70s.
I went to a regional state university. We had an older IBM mainframe with a hypervisor and the students and faculty were all users on MUSIC/OS. This was in the early/mid 1990s.