d20 backlash.
I guess I dont quite know what that means. Backlash by who? I think the answer to the question is different based on who the "who" is.
Backlash by purchasers?
I wouldnt call it a backlash, but I see what you are saying and I have witnessed that. The initial flood of products certainly contained a big batch of crap along with good stuff, and people got burned. I think it has been more of a "whittling away" by purchasers. People have come to know who they can trust to deliver the goods adn I think people have tended to let that control some buying decisions.
The other issue for purchasers is marginal utility. For example, when 3.0 first came out adventures were all the rage for two reasons: 1. no one had any and the WotC ones were lame so people needed them; and 2. they were (relatively) easy for publishers to get out right away. But at some point, their marginal utility decreased. As Dungeon got better at hitting the target and as people got more comfortable making 3E adventures themselves (that was a learning curve for DMs, since 3E is WAY more preparation intensive) there was less need for 3rd party adventures. Heck, if you have 10 adventures already, what is the marginal utility of getting the 11th? Low. BUT, if you have 0, the marginal utility of getting that 1st and 2nd one is very high. Same thing with rule books. That first book of spells was useful. But the 6th one isnt that useful, unless it is really good.
And that is the problem. Marginal utility. At this point, most things have been done. Now it is just down to doing a "different one." People want the first one, but dont necessarily need the different one. If you've got 3 setting books, you may not need the 4th (though frankly everyone needs our Wilderlands setting, nothing in the world is like it

, but I digress...).
Backlash by gamers in general?
Sure. Its just natural gamer contrariness. I have it. Every gamer has it. Its why we all left AD&D for GURPS or for some other system. We just get bored of the same thing after a while. "Lets play something else." Are people a little less geeked about d20? I'd say yes. Plus there are some people who just want to hate WotC and D&D and that is just that.
Backlash by stores?
Yeah, maybe. They got burned by some early d20 too. And they tend to follow the money. That is why all the publishers went to hardbacks, because the retailers really felt comfortable ordering and stocking those and were less comfortable with softbacks. This is particularly true for bookstores (as opposed to game stores).
Backlash by publishers?
I dont know. It hasnt happened to us. We have always had a market strategy and have tailored it over time. But we have always had one. For the companies that just jumped in for the jack then they are jumping out. Or maybe things didnt make as much money as anticipated. Whatever. We (Necro) have had no trouble maintianing our amount of releases over time and have stayed pretty consistent and had no problem. Sure, sales numbers have dipped (oh for the days of Crucible of Freya!) but they hit their point and have been pretty consistent for some time now.
There are lots of reasons why companies fold up, many that have nothing to do with d20. d20 was a good entrance for new companies, and new companies fail for all sorts of reasons, many ahving to do with people and interactions and maintaining and interest level and work ethic to get projects done; all of which is not related to the health of d20 at all. Sometimes it is financial, sometimes personal, sometimes motivation, sometimes a bad business break. Whatever. In fact, you should expect more failed companies in a field that is "entry level friendly" which d20 certainly was for a long time. And company failure often has nothing to do with quality of product. I like to use Fiery Dragon as an example. If I had my way (and I did for a while

) those guys would be on top of the world. I have always loved their stuff and the quality of their products. But something just didnt catch on and work for them as a book print company. But they have been super creative with the counters and the battle box and other things. It is that same creativity I love in their products that I think has kept them going. But if quality and merit alone were the reason for business success, in my book they would be the richest guys in gaming.
But back to my point, it would be hard, without a very specific focus and plan, to be d20 only in my opinion. You dont see many people doing it any more at a real successful print product level. And the ones that are, IMHO, do have that focus. We've been very lucky in that regard. Unless your name is Monte Cook, it is pretty hard to be a d20 only company.
Just my thoughts...
Clark