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In my experience, D&D was not what I would call "dominant" in the 1970s-80s.
If DnD was not "dominant" in the early days, then "dominant" was not the word I was looking for with the poll. This might explain some answers I found weird, where people seemed to equate dominant as the ONLY acceptable game.
The kind of "dominant" I was looking for was the one used in the "Empires in Arms" game describing the situation in Europe in 1805 to distinguish major powers (The Ottoman Empire, Prussia, Austria, Spain Russia) from dominant powers (France, Great Britain). That is first-rank with a significant lead on the others, but by no means the sole player.
If "dominant" is widely understood to mean something else, it was a bad choice of words for the poll - I am not a native English speaker so its the kind of mistake I could make. I did explain myself in a later post tough.