Unix wasn't a mainframe OS to begin with. It was a minicomputer OS which eventually scaled up to mainframes, but it was still born on minicomputers and retained

Unix wasn't a mainframe OS to begin with. It was a minicomputer OS which eventually scaled up to mainframes, but it was still born on minicomputers and retained a minicomputer design: Character terminals as opposed to block terminals, no channel I/O, and batch support sidelined as opposed to front and center. VMS also wasn't a mainframe OS: It ran on the VAX, a high-end minicomputer. Looking at what early microcomputers looked like and what their OSes were based off of (CP/M in particular) it would be more reasonable to say that early microcomputers were scaled-down minicomputers, which were still in a different league from mainframes in terms of number of concurrent users, I/O design, and focus on interactivity.