Are you sure we haven't converged on a good thing?
Before Unix (and let's be clear, Windows NT uses basically a Unix process) there were lots of process models. VMS was very different, CP/M and MSDOS different than that. My mainframe knowledge is limited, but IBM's VM/CMS and Burroughs' Bxxxx machines were different still. Mach, what's underneath MacOS, is quite different conceptually. All those have fallen by the wayside, because of various inadequacies.
Before Unix (and let's be clear, Windows NT uses basically a Unix process) there were lots of process models. VMS was very different, CP/M and MSDOS different than that. My mainframe knowledge is limited, but IBM's VM/CMS and Burroughs' Bxxxx machines were different still. Mach, what's underneath MacOS, is quite different conceptually. All those have fallen by the wayside, because of various inadequacies.