Many prestigious, critically-acclaimed original shows don't actually garner big ratings, but make up for it in other ways.
FX helped kick off the trend of serious original dramas on basic cable, so it's more tolerant of smaller audiences, up to a point (as anybody who recalls Terriers is probably well aware).
While many point to The Sopranos on HBO as the beginning of the current Golden Age of TV, it was on pay cable.
FX (with The Shield) and AMC (with Mad Men) shouldn't be ignored, and they've carried on with that theme, with Breaking Bad, Saul, Damages, Rescue Me, The Americans, Justified, and so on. I'm sure I'm forgetting some, but the ones cited are highlights.
I consider USA differently because it hasn't been in that game nearly as long as it has been the "Blue Sky" network. Mr. Robot changed that, and bringing in darker shows like The Sinner was part of the pivot, but I'm not sure how committed it is to that direction. Matt Bomer played a very different character on White Collar than he does on The Sinner for USA.
One could argue that FX is the HBO of basic cable. AMC could be in the same conversation, but lacks the consistency. USA is a relative newb and hasn't displayed the same history or commitment, which is why I question The Sinner's future.