[fjtorres]

Bookstores getting encompassed by general-purpose retailers?
Short-term, yes. That part of the business dedicated to the High Volume "Bestsellers", for sure.
Long-term? Maybe not.

If ebook mainstreaming reaches a large enough volume at its peak and print book volume drops enough, the minimum-margin/high-volume retailers might just drop out of the business.
Barnes and Noble has openly stated that they expect to outlast the general purpose retailers and be the last B&M book retailer standing. They may be fooling themselves or they may be looking at a vision of a "bookstore" markedly different from the current version. Either is just as likely.

What makes Bestseller books attractive to Target and Wal-Mart is the foot traffic they draw in and the ease of sale: set up a rack, refill as needed. Decent dollars per square foot result with minimal effort as the books sell themselves. No marketing, no customer questions. But if the volume drops because enough people start buying ebook versions it won't take much of a drop before the ratio drops to a point where either the discount goes or the books go.

We really don't know the overall size of ebook reading's natural market.
We don't know how many people will refuse to buy anything but print.
We don't know how much of a decline in print runs will take to make batch-printed "Bestsellers" stop being profitable at high discounts and we don't know how bestselling those hardcovers would be without the high discounts.
And we don't know at what point withering Bestseller discounts will start driving buyers to ebooks instead.

It is way too early to know exactly how the endgame will play out; all we know is the general trends point to a tipping point that could suck the profitability out of the commodity book business long before demand dies out and that at some point (soon?) print books will slide into a death-spiral of rising retail prices and declining sales.

The beginning could be as simple as the BPHs applying the Agency Model to print books.
That kind of price fixing would surely prop up bookstores (in the short-term) and might push away the discounters. It would also drive a lot more readers towards ebooks.

In that, I think the idea that the travails of B&M book retailing help speed ebook adoption is probably right even though ebooks are not the root cause of those problems to start with. (In evolutionary terms; mammals didn't cause the demise of the dinosaurs but they were merely the best positioned to take over once they did start to decline.) The same applies to books: as the giant chains decline, leaving entire regions bereft of large-catalog B&M print retailers, the volume handled by online retailers and ebookstores will inevitable go up.

And, given that readers tend to stick to ebook buying (for the most part) once they transition to ebooks a pbook death-spiral into niche-dom is pretty much inevitable.
It's all a matter of time.