Yup, if it wasn't Microsoft, all kinds of other companies could have dominated the desktop market. IBM (OS/2), Quarterdeck (DESQview/X), Apple (Mac OS), NeXT (NeXT), any number of *nix companies (X11), and others.
Microsoft got big because they got the consumers interested, and questionable deals with vendors.
Plenty of people only know the tunnel-vision version of computer history and they believe Microsoft is it. They either don't remember (or are too young to have seen) software boxes (ahh, the good ol' days) had logos to indicate which OS they worked on so you could pick the right one.