It's great that Netflix finally beat HBO's subscriber figures but given that subscriber figures are cyclical (though HBO's have evened out now that HBO Go is out -- which incidentally was largely the impetus behind HBO GO, to curb subscriber churn during period's when new series aren't on), it'll take consistent effort and good programming for Netflix to remain in this position.
The problem is, that strategy takes money. And that $8 a month per household is probably half of what HBO is getting per household for its 29m subscribers (plus what they get from home video, licensing and international).
Netflix has pivoted from trying to be cable to trying to be HBO. That's all well and good but let's not try to spin this as how cord-cutting is "winning." It's not. If cord-cutting was winning, Netflix would have had more subscribers than HBO quarters ago. Instead, Netflix's next big strategy is to get bundled WITH cable providers (which incidentally is a smart approach and could be what really lets Netflix's subscriber base balloon).
HBO, Netflix and Showtime require original content to really push the needle. HBO has an advantage of having spent the last 20 years (since Larry Sanders, really) cultivating the best showrunners and writing talent that have in turn, brought in some of the best programming in the history of the medium.
Moreover, HBO has a financial advantage in that they make most of their original series in-house. Netflix does it two ways. They fund series outright like House of Cards and Arrested Development (but to be clear, 20th Century Fox still produced and owns the video and syndication rights to AD) or they acquire the first-run rights (Lillyhammer, Tom Fontana's Borgia). Even in the cases where they do fund the development, a secondary production company usually still does everything -- it's not in-house. HBO, by virtue of being a subsidiary (and the most profitable subsidiary) of Time Warner, has access to Warner Bros. Television, which cuts down on a lot ancillary costs before it comes time for distribution.
It'll be interesting to see if Netflix can afford to acquire or do full-contracts with certain production companies and creator shingles to get rights to future projects. Alan Ball has a longstanding deal with HBO and David Milch has a semi-exclusive with them too.
TL;DR This is great for Netflix, but all it really does is make them a real contender as a premium content network. It doesn't impact HBO's current economics or have any impact on the realities of running a content business. Which is basically that good shit is expensive.