Yeah, I was just about to ask if we should set a definition of mainstream, only to find that Cheesegear's "mainstream" 40k thing would have to be the most popular and famous thing in all of human history, shattering all known barriers of language, spacetime, culture, interests, and socioeconomics.
So the answer to the question is, it was never going to happen. Not because of anything to do with 40k's viability as source material for popular entertainment, but because we're describing something that has never existed and likely never will.