Was it a successful launch? What metric do we have that says this?
I think the relative lack of edition warring this time around is all the proof of success they really need. They have a moderate version of D&D that no one hates enough to declare a ceaseless campaign of vitriol, lies, hatred and misinformation against. That's all they need to halt the corrosion of the property. They're back to comfortably resting on D&D's 40 year history and first-RPG laurels.
With no meaningful potential for growth in the RPG market ( from the little I've heard, it may even have contracted in the last 8 or 10 years - I recall a 2005 or 2007 or so estimate of 20-25 million, today IcV2 seems to think it's only 15), and the D&D legacy no longer under active assault, they're thus free to try to grow the franchise in other areas where there is potential for growth. They can fight tooth and nail over the larger half of a 15 million dollar market (and possibly start tarnishing the brand image again), and make a few million more (compared to the hundreds of million CCGs rake in), but if they can finagle even a tiny slice of the MMO market or get even one modestly successful summer movie out there, it'll bring in tens of millions.
Sure, but 4E sold really well at first as well. The topic at hand is not what is happening with sales of 5E in 2015.
4e stayed in first place for 2 years, even while Pathfinder released it's core books. Essentials crapped out and lost the top spot. It took a quarter, but 5e has D&D number 1 again, and it's not even trying hard. The lesson to take from that is don't go screwing with your product and your design goals too soon, no matter what geeks on the internet have to say.
The topic at hand is how do you maintain the brand value of D&D long term (or at least does low release rate have any impact either way).
I'd guess that D&D's long term brand value is not based on it's TTRPG sales in a give quarter or year or even decade, but on its status as 1st RPG, and it's mainstream name recognition. Neither of those are threatened by a slow pace of releases. Neither were in the least damaged by a two year hiatus, for that matter.