I remember them, but they cost a little fortune (both hw and sw). They were still popular in the 90s but the first cheap and good enough alternative started to make a real dent into their professional market. It was Windows 3.1 and 80486 in 1992. But don't forget Autocad in the 80s.

Yes, CP/M-86 could have played to role of MSDOS but people really needed to buy a box with a floppy disk to install from or a preinstalled OS on their new PC. MS did that right.

A 386 was a reasonable Unix host at the time. I first experienced Unix on a 68020 box attached to dozens of terminals. There were a couple Unix like OSs that were relatively inexpensive but I don't remember any that had a GUI.

All that is not very relevant. Had Microsoft never existed, it'd have been an ecosystem too different to make any valid prediction. We'll have to eventually restore the universe and run a different path to see.

That "POSIX compatibility" was quite interesting in the late 90's.

Just like it happens with any standard, we ended up with lots of #ifdef, because many APIs might had the same name, but not the same semantics.

And like in UNIX like OSes, all the cool goodies weren't in the POSIX APIs anyway.

It is quite sad that despite some of the cool GUIs like on NeXT and Sun OpenWindows, the best that they could settle on as a standard was Motif.