Actually PC-DOS was the defacto standard. MS-DOS had smaller marketshare.
Were there lots of other operating systems?
You bet!
There were over a hundred of them
Really? Besides CP/M and DR-DOS how many can you name.
Truly delusionary.
Perhaps you should do a little homework before you make idiotic statements about "hundreds" of PC operating systems. If your are talking about minicomputers & mainframes, they are all Unix derivatives (also open source).
Before you ridicule open source, perhaps you could consider that the ENTIRE internet is built on "open source", and includes the specification for HTTP & SMTP/POP3, DNS, et al.
Or did you think that Al Gore worked for Microsoft?
IBM and AT&T once had 80%+ marketshare too. International standards encouraged the competition that trimmed them down to size too.