GEOS, GS/OS, and GEM should've all been more popular and longer lasting GUI systems than they actually were.
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I still think of what would've happened to CP/M-86 and GEM if IBM had put in a modicum of effort to meet with Digital Research instead of Micro-Soft.
@arielmt on the other hand, if DR had had CP/M-86 ready in time, 86-DOS (and thus MS-DOS) would never have existed in the first place