It's worth pointing out that D&D is an altered and commercial version of Dave Arneson's Blackmoor.
True, but Arneson's Blackmoor was, basically, a bunch of variably organized house rules to Chainmail on scraps of paper, index cards, and sticky notes that are maybe organized in a binder, and Arneson, from what I've gathered, didn't necessarily feel like organizing it into a commercial product (and considering his later track record for deadlines, and how well he did without an editor or collaborator, even if he did that didn't mean it'd come out in a playable form).
So, what we could have gotten instead would have been various Braunstein games getting written up through after action reports in fanzines and semi-pro zines, inspiring people to try to start their own similar Braunsteins with varying degrees of success.
Possibly the first game to jump into being a "Commercial Braunstein" at this point might be the Star Blazers miniatures game, with the framework of the first season leading itself to that structure, or a Star Wars or Battlestar Galactica game.
Where, in this more Sci-fi inspired alternative history, things go to something closer to a proper tabletop RPG as we think of them now would be something designed to kind of haphazardly do both Star Trek and Star Wars styles of play.
Ironically, in this setup, Gangbusters still comes out, but as a game designed solely for Braunstein-style play.