If most people using a product are doing so in the same way (which of course could be untrue of many ttrpgs to me), isn't something that dominates market share definitionally mainstream?It relates to my concern very much, because it PERFECTLY illustrates the absolute arrogance associated with a large segment of RPG fandom which seems to consider itself the absolute arbiter of what is acceptable, mainstream, etc. That poster simply dismissed not even a genre or classification of RPGs, but essentially everything not produced by those he seems to favor as irrelevant trash with a metaphorical wave of the hand.
EDIT: On further consideration, "among those participating in or familiar with that market" anyway. If it's a small market, then not necessarily among folks at large.