Hitting any non-fiction bestseller list isn't that hard. For April 2nd, when the PHB2 was #14, "Who Moved My Cheese" was #15, and it's an eleven-year-old book.
I just find it amusing that the D&D stuff, being about how to play in a fictional world is classified in the non-fiction category.
I'm not sure if we are close to the end of D&D as an entity. Deaths usually are either going out as a whimper or one last supernova burst before dying out. But a lot of other death scenarios are possible.
I think Wizards's input into D&D is on a death-spiral, considering how and why things are being produced. (As an aside, I find it interesting that Stan! and others have commented on how Wizards has produced way too much material, where I would contend that they are produce no where near enough considering how the production came out with 3rd edition and 3.5. It's like one rule book, one module, and maybe one or two tie-in novels per month instead of the multiple books and modules that they usually get. But that's neither here or there.) If you compare the timing of when they are producing books versus when they were producing similar stuff for 2nd and 3rd editions, I think you'll see that they've sped up when these things ought to be published, which is both a blessing and a curse for how they decided to organize and market the eventual complete game set.
I agree that the solution is to slow down on production, but not to slow that you inadvertently wean the customers off of your crack, er, product. Ever notice that WoW will produce a new chapter once every year and a half? They're really good at getting people hooked on something, and just when the thought of that they are almost getting tired of the game sets in, hey, here's an expansion set. And then they don't mind the half-day wasted downloading and upgrading their stuff. That's what Wizards ought to do.
And then they can be less concerned that piracy is taking a good chunk away from their presumed market share.