Two quibbles: first, I'd argue that DOS, Windows 3.x/9x/ME, Windows NT are three separate OSs. Ditto MacOS and OSX. Secondly, I think we're in agreement about

Two quibbles: first, I'd argue that DOS, Windows 3.x/9x/ME, Windows NT are three separate OSs. Ditto MacOS and OSX. Secondly, I think we're in agreement about the 'dominance for decades' thing. I should have said "Very, very hard ... and will take a long time." Think of ITS, Symbolics, VMS, HPUX, Solaris, PalmOS, CP/M, OS/2 ... all once popular, now consigned to the dustbin.