The d20 crash was when WotC produced 3.5 without warning, leaving d20 publishers, distributors and stores with large amounts of d20 products that were largely unsellable. I don't know what the impact was on WotC, but it's not directly about WotC.
Which would explain part of the reason why 3.0 PHB is available for 5 bucks on Amazon and the 3.5 PHB is still running 15. But I never saw anyone sticking with 3.0. I'm curious what the numbers were for the DMGs, and if they were substantially different. Also, I was under the impression the 3.5 splatbooks sold a lot better then the 3.0 splatbooks, the d20 crash (having driven competing splatbooks out of the market) and making them hardback apparently helped that.
Yeah, all these figures you see online are dubious. I read in an interview with one of the former WotC guys that the 4e core books sold more than any previous edition. Of course he may have meant implicitly "right out of the gate" or something, I don't really recall the exact wording. But that's it see, things are remembered, memory is unreliable, maybe I remember it exactly right, but maybe the guy that said it didn't know or misremembered or lied, or maybe there was some subtle qualifier in there, maybe even just contextual.
So, the Eric Mona figure COULD be correct and the quote I read could be correct and the problem was simply sustained interest in the face of things like PF. I gotta believe PF hurt D&D a good bit. It was JUST enough of a refresh that a lot of people bought it even if they were basically happy 3.5 players, and its really rather hard to find someone that wouldn't quickly and painlessly migrate to PF without a big ruckus. You can take your 3.5 group and nobody will get all steamed or feel unhappy about learning a new game, its just not really that new.
Obviously that begs the question about what WotC should have done, but I think the problem there is twofold. First is the NARRATIVE, "plucky 3PP left out in the cold strikes off on its own to help players left behind by nasty WotC 4e moneygrab" vs "Nasty WotC 4e (albeit 3.75) money grab, why bother?". They just couldn't win there. Releasing a 3.75 for WotC would have been stillborn. It would have sold some books, but it would have been a slow and hard sell, probably slower and harder than 3.5, not what they were after.
The point being most of the people that bought PF wouldn't have bought 3.75. OTOH producing 4e wasn't a better solution. I'll give Mike credit for one thing. He has shaped the anti-4e backlash into a more sympathetic narrative for WotC. Now its the "poor bumbling fools have learned from their mistake" narrative.
However, they still face the OTHER WotC problem. WotC has it would seem driven out all the really creative talent they ever had in house. At least they somehow don't UNLEASH it creatively. I think things are just too much "brand management" over there. When Paizo releases a product its cool. When WotC releases a product its much more bland.