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?
I can only think of two possible reasons:
One is the subscription model was of paramount importance and WotC couldn't/wouldn't make use of such a sales mode. Which isn't surprising, since that's why they offloaded the magazines to Paizo back in 2002. I'm still curious if it was a lack of means or a lack of will, but those are really the same thing, since you need both to make something work.
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.