I'm just one single buyer in this specific market, and I don't think I'm in any way representative of other buyers' habits. But as this thread asks for my own personal part in "destroying the industry", I'll give an answer

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The first point is that, after the release of D&D 3.5, my buying per time period went down to roughly one third of what I bought in the 3.0 days. I bought the 3.5 core books as replacement set with errata, but what I got was a new version (especially in the spells section). Suddenly, all d20 books wore a "Best before [put in date 3 years ahead or less]" tag for me. Of course, I know they don't turn bad, but they need conversion = work. It's a psychological thing. I didn't buy anything at all for somewhat more than half a year after that, and when I started buying again, it was only stuff that was decently discounted. The abovementioned ratio of one third is true for my WotC purchases. 3rd party companies differ vastly.
The second point is that I don't need that much stuff anymore. Perhaps, "need" is the wrong word, because I didn't really need most of the books before, either. But it's pretty obvious for me now that I will never be able to use all the stuff I already have*. This means that new material has to be more intriguing than during 3.0 times, when everything was new, and this in times when good new niches get rarer and rarer.
On the other hand, I buy slightly more stuff for other games than D&D/d20, but never complete lines. It's definitely not enough to compensate for my general buying decrease.
*I suppose this point differs for people with vast libraries from 2E times and before. I can imagine that they often did not buy that much in the first place when 3.0 came out.