How Mainstream is Dungeons and Dragons with 5e?
D&D is the same kind of 'mainstream' it's always been: the only TTRPG with significant mainstream name recognition.
It's also moving units like it hasn't since the end of the 80s fad (OK, books aren't flying off the shelves as fast as they did then, but it's a different book-publishing world, out there).
This isn't a great factoid, but it's pieced together from credible enough glimpses, and rounded off to make it look good:
3.5 moved ~375k player's handbooks over its run and was counted a rousing success compared to 2e.
5e had moved 750k books by 2018.
TRS was moving 750k books per year at the height of the fad.
Observations: There was no streaming, on-line virtual table-tops, open-source d20, character-builders, or pdf piracy back then. There has been surge in boardgame popularity for years, starting not long before 5e came out. There actually /was/ a sort of edition war back then, in the years before the fad got into full swing, over 1e vs 0D&D, and the PF analog was a game called Arduin Grimoire, that TSR sued.
Speculation: WotC's D&D brand is probably making more money than TSR did back then, because, sure, inflation, but also because it's not being mis-managed.
Unfounded conclusion: D&D's never had it so good!
Is now the time for the coup to finally remove the hated Paladin class?
There will never be a better time.
Maybe later.