First, people who semi-regularly buy one month subscriptions are still paying WotC money, so at worse they're partial successes from WotC's perspective.
Second, people who had subscriptions and have let them lapse are likely to be a small minority since yearly subscriptions, probably the most popular amount, won't yet have lapsed.
Third, if all this does is count whether someone is a subscriber at the time they visit the new forums, then it won't have much error in it yet because the new forums haven't existed for very long. I think less than a month, right?
I will agree with the underlying point that, over time, this will become a less and less valuable measure of the DDI's base, and that right now this doesn't tell us how many year long subscriptions have sold, just how many individualized purchases have been made by forum members. But a purchase is a purchase.