I concur that Hasbro is a/the big answer. The "50M or die" dictum and that WotC was being led by someone who wasn't a gamer (and instead came from boys toys) was not a great combination for an RPG maker. Especially when it drove unfriendly fan policies (a repeat of TSR's foot shooting, in many ways). Plus, IIRC, I read somewhere that when WotC was purchased, Magic and D&D were considered separate divisions -- had they not been, Magic alone would've carried the division, and WotC could've let 4e's modest (by comparison) revenue continue indefinitively, and let passion rule the day rather than "profits".Right, but why? Why could Paizo make that model work – getting over a decades' worth of additional life out of 3.X – where WotC couldn't?
The second possibility (and, I think, the much stronger one) is that the revenue brought in wasn't sufficient for what WotC wanted. The salient point here is that there was still enough money to be had to keep the lights on and everyone paid, at least for a company Paizo's size (which then leads us to ask whether or not the D&D part of WotC is comparable to Paizo in terms of employees, space, resources, etc. that they need to pay for). Rather, the revenue issue is less about solvency than it is with hitting Hasbro's target numbers; remember that according to Ryan Dancey, this is back during the era of "Core brands" that Hasbro wanted to earn at least $50M, and preferably $100M+, per year. Earning less than that doesn't mean that a venture isn't profitable (i.e. makes more money than it spends), but rather means that it isn't profitable enough.
All of which is to say, the idea that a Paizo-style subscription model for 3.X was "unsustainable for WotC" means (as I see it) that it likely never would have been able to meet Hasbro's demands. Whether or not it could have been sustainable in terms of keeping the D&D section of WotC in the proverbial black strikes me as a very different consideration.
As for book sales, one thing that skews numbers are the digital subscriptions -- I bought a bunch of books early on (by my quick count I have 12 on my shelf), but later relied on my DDI subscription to get all the bits that I wanted.
And as for page counts, don't forget to include the monthly Dragon and Dungeon issues. A good and sizable chunk of game material (class options, feats, and more) came from Dragon!