DaveMage
Slumbering in Tsar
We can pretty much infer that 50-odd thousand subscribers means 50-odd thousand people interested in playing 4e (even if they aren't currently doing so). And we can assume it generates something in the neighborhood of $500,000-$600,000 per month. What it costs we can really only speculate at and so we don't have much information on net revenue. I presume it's profitable and probably helps turn big troughs in the revenue stream into a more steady inflow. That's good for WotC. But I have no idea if it also blunts the spikes in the revenue stream that you'd see with regular published offerings and I have no idea how far it cuts into their book sales (I'm reasonably confident it cuts to some extent). As a result, I don't know if the net effect is good or bad or how WotC interprets the results they are seeing.
Well, I think WotC's intent was to produce a core set of evergreen books with the occasional supplement here or there, so I don't think they really see the DDI as cutting into sales. Rather, they see the DDI as their D&D RPG revenue stream. Of course, the D&D *brand* includes the board games, accessories, novels, etc. The brand itself seems quite healthy, even if the core RPG physical book sales are slow. (In other words, they now only have to put out the occasional RPG supplement. This could actually be a good thing for 4E fans because WotC will - hopefully - choose such titles carefully, and thus put greater emphasis on quality and value.)