“Microsoft bought DOS (86-DOS, a.k.a. QDOS) from Seattle Computer Products for a measly $50K, re-branded it MS-DOS, and licensed it to IBM as PC-DOS. All perfectly legal business.”
That’s a completely dishonest statement of what happened since Microsoft added significantly to the code base to make it MS-DOS.
Incomplete yes, dishonest, no. Microsoft hired Tom Paterson to port 86-DOS to the IBM-PC, and with their knowledge of how operating systems ought to be (from XENIX), certainly added a lot to it.
For example, if you were around in those days (as I was), you might recall SWITCHAR, one of many modifications to make the CP/M-like 86-DOS more palatable to the Unix-heads that populated Microsoft’s development teams.
To this day, both forward-slash and back-slash are recognized as path separators, even though COMMAND.COM, CMD.EXE, and the Windows GUI only recognize back-slash.
Microsoft developed MS-DOS considerably after they acquired 86-DOS. Saying so wasn’t in my original comment because I was only trying to counter the false accusation that they “stole” it. You are correct to point out that they also added much to it.