So we have concluded that the DDI subscription numbers tell us how many people are playing 4e? At least close enough that it dominates any assessment of the marketplace?
I wouldn't say it's one-for-one, but I would say that it's probably a big factor (and I say this only from Truthiness, not from any actual factual information

).
Y'know how 3e's big business model was to drive people to buy Core Rulebooks? You sold supplements, and you used the d20 System, to attract people, ultimately, to your PHB, which you sell over and over and over again.
I think 4e is more trying to drive people to the DDI. The supplements and the PHB's and the constant upgrades and the yearly model all funnel into the DDI. This also plays into a "no OGL" way of thinking: they don't want to have to add everyone's content to the DDI every month (not that they do now) or have anyone make a DDI clone that does it better than them (not that they do now).
Like the 3e Core Rulebooks, not everyone who plays the game gets a DDI account, and not everyone with a DDI account is an active player, but I think a reliable monthly subscription largely takes the place of steady core rulebook sales as a "baseline" for D&D income.
The supplements, the card games, the board games....these are all additive.
Of course, given the rumor that WotC was kind of hosed on the cost of the DDI so far, and given the lousy economic state of things, I think 4e, like every other industry, is under a lot of additional pressure to make sure those additive purchases pay off. But I see a DDI subscription as analogous to buying a 3e Core Rulebook Set. Over the course of time, it actually adds more to WotC's coffers, so it's a pretty smart move, if that's the case.
Again, this is all Truthiness, not Truth. For all I know, DDI subscription rates could be more like Kindle book sales than like 3e Core Rulebook sales (that is, only the truly ardent fan, or a special niche market, bothers with them). But even then, returned 4e PHB's could be more a symbol of an ongoing 4e player than a converting 4e player, given things like Essentials and the Rules Compendium and the constant, repeated rules updates making much of the material in the book invalid anyway.
Speculation upon speculation!
