I'm having trouble understanding your logic, which seems flawed. Let's expand the example a bit. According to one estimate, 320 million people on Earth speak English and are literate. According to your argument, any book that does not sell 53 million copies is a failure. The last Harry Potter book sold 11 million copies -- only 1/29th of the believed market bought the book. Was it a failure?
That borders on hyperbole, but I'm sure you see my point. I'm just not sure your metrics are accurate or realistic. Let's put it this way: I am a huge fan of Paizo, and I greatly hope Pathfinder approaches those sales.
I'm not surprised the PHB2 has sold out. In publishing, you get significant discounts based on bulk printing, but those eventually max out. I imagine WotC's initial printing was likely at the maximum volume discount. Due to the cost of warehousing and carrying inventory, it's smarter to make multiple printings once you hit that point.
WOTC has been called the market leader for years, you are not the market leader when you can't even attract 1/6th of the market that you claim exists ("you" being WOTC, they are the one that touted the 6 million "D&D" players world wide). So now we find out that their core books of their touted 4E hasn't even been sold to a full 1/6th of the "estimated D&D market world wide".
So now we finally have solid sales numbers (solid in that we know its "hundreds of thousands", which is less than 1 million), which now means we know that 4E appeals to less than 1/6th of the estimated 6 million gamers that are out there.
So now I would really like to know Paizo's numbers. I bet they are in the 10's of thousands, but if they are actually at 100,000+, then WOTC is not as big of a leader as previously believed. Especially since "Hundreds of thousands" could be as little as 200,000.
So this could actually mean "D&D" as a brand is much closer to being "toppled" than previously believed. It has all kinds of possible implications.
I actually think WOTC is much closer to 1 million, but thats largely because I have been believing their hype. Now if WOTC has been over exaggerating their success to keep the perception of being an unbeatable juggernaut alive, you know, like AIG and so many other American companies were caught doing about 6 months to a year ago, and are now going out of business, in danger of bankruptcy, etc... then the whole perception of the D&D brand will change, and become seriously devalued, just like our housing market and economy has become devalued.
So yes, this has serious possible implications for the D&D brand and WOTC.