It's interesting that Microsoft were actually behind the curve as it was. Early Windows did indeed sit on top of DOS, and v.1 & 2 just did not work. V.3 .0 was the first version that worked. But…
In the 1980's, I was system manager for a multi-user, multi-processor system using Z80 8 bit microprocessors and a version of CP/M known as MP/M. This system allowed for up to 14 simultaneous users, upto three public printers, plus each user could have their own personal printer. And in addition, it was possible to have up to 2 dial-in lines. The system used bank switching to allow up to 256kb RAM. (A Z80 was designed to access 64kb RAM.) And all before Windows appeared.
Then, in 1989, I moved jobs within the organization, and gained a PC AT for my work. This machine came with DOS 3.20, 3 versions of BASIC, and a program called Desqview along with 2Mb RAM. Once set up correctly, Desqview allowed task swopping, and for the copying of data between programs. Yes it was a bit clunky, but it worked. Again, all before Windows appeared.
Finally, at home in about 1990, I ended up with a Sinclair PC200, a PC XT clone running DOS 3.30, and if desired a program called GEM which allowed for two windows to be open simultaneously to allow easy manipulation of programs, eg copying, moving etc, but not task swopping etc. Again this was before Windows gained its stranglehold and introduced the general public to these ideas.
I have to say that Microsoft managed to combine all the above ideas more or less successfully into one program and they deserve credit for it. Unfortunately, they then went on to, in my opinion, abuse their position, witness the anti-trust cases and the fines levied against them.
Peter G. Shaw