EN World has an impact, and it's really huge, but it might not affect the RPG business the way you think it does.
In terms of raw sales numbers, EN World has no noticeable impact. If anything, there seems to be an inverse relationship between how often people talk about a book and how many copies it sells. The character of conversation ("This sucks!" v. "This rocks!") doesn't seem to be a good indicator of how many copies a book moves.
My personal theory is that book that sells lots of copies has a sizeable number of supporters *and* detractors. This shows that a broad number of people are buying the book. You can't sell 20,000 copies of an RPG book without at least some of your customers disliking it. When you have a product that gets only positive comments, chances are it's a niche product that appeals to a small group. Everyone in the small group likes the book, but no one outside of that small audience has bought the book.
Thus, I think the Internet suffers from a chicken and egg dilemma. The Internet doesn't make a book a hit, but it can give you some evidence to see what people are buying.
While EN World has no real effect on sales, every d20 publisher of note reads and pays attention to EN World. To a lot of publishers, praise on EN World (or RPG.net) is a huge boost to the ego. I think that opinions expressed here have a big impact on how many publishers do business. If everyone on EN World started talking about how much they wanted a steampunk d20 game, I'm sure you'd see a publisher do one.
This makes it doubly funny when a troll posts something like, "I hate company X! They're a bunch of piss midgets! Their books blow!" People from company X are quite likely going to see that, but if you give them a good excuse to ignore criticism they'll likely take it.
All bets are off when it comes to PDFs. EN World *is* the PDF market.
