An interesting thought. One aspect is that--while some of these games were attempts at 'my take on the D&D formula, or 'D&D, but with/in _____,' in many ways a lot of these other early games ceded specific ground to D&D. That raises (to me) the question: if D&D had not been, what would ____ have been like? I think quite possibly if D&D hadn't been around, RQ would have had a more 'Tolkien-esque/traditional ren-fair medieval milieu. Likewise, I think if Traveller had caught on big, it quickly could have had licensed Star Wars and Star Trek ports.I suspect EPT and probably RQ as-was were too exotic in their extent settings to make that work. With Traveller its always the question of whether an SF game, especially one with the kind of merchants and mercenaries focus Trav had could do it (which doesn't mean it couldn't, just that you can question it). Arduin wasn't really a standalone game system that early. DQ--maybe. It was more complicated than OD&D, but perhaps not any more so than AD&D.