...Thinking about it, I suspect discontinuing the skirmish game may have killed the line.
It's kind of an evolutionary thing. Collections of traditional minis are driven by evolution relative to the environment only--with the "environment," in this case, being the fantasy world modeled by the game. Buyers will continue to "evolve" their collections by purchasing new minis as long as there are places where their collections are inadequate to model the game world. Eventually, though, they reach a point where their collections are so well "evolved" that further adaptation is not worth pursuing.
The skirmish game, however, added a competitive element, allowing miniatures collections to evolve relative to each other. No matter how good your collection is, there's always room to gain a new edge over the next guy's. With a slow but steady drip of power creep, or something like M:tG's non-eternal formats acting as a kind of planned obsolescence, you can drive steady sales more or less forever.
Apparently the skirmish game wasn't popular enough, though, so it got sacked; and now sales are being driven purely by the shrinking pool of collectors who aren't yet content with their sets.
On the plus side, if DDM does die off, there's a good chance 5E will move away from miniature-centric mechanics. A combat system heavily reliant on miniatures is good if you're trying to sell minis, but if you aren't, then miniatures-dependence is just another barrier to entry in a hobby that already has more than its fair share of them.