They might be why you subscribe (and I like Silicon Valley, too) but they're not why customers in general subscribe. True Blood had something like 6x as many viewers as Silicon Valley (this is what I mean when I say HBO's business isn't really "quality" so much as stickiness); for Game of Thrones, it's more like 12x.

But also, you can't look at those shows in isolation. Compare them to Netflix's properties. Stranger Things is as big a deal as literally anything HBO has launched in the last decade, including Game of Thrones (I really want to believe that HBO is kicking themselves for not buying the script for that; it was shopped to them). For any given HBO 30 minute comedy, Netflix has probably launched 3 equally credible comedies.

The very best, most important 60-minute scripted dramas in the past decade have not as a rule, with the exception of Game of Thrones, been on HBO. If AMC can do Mad Men and FX can do The Americans, there is no reason to believe that Netflix can't do equally well just by pouring investor dollars into acquiring shows. At the end of the day, that's what these businesses do: pick production companies to fund, fund them, and then distribute the results.