I'm starting a career in book publishing, and this debate is all over the place. For now, sales of ebooks vs. physical copies has mostly leveled out.
I don't think physical books are going to disappear. A lot of people thing ebooks should be waaaay cheaper than a physical book, but unless they're on the inside of the industry, the average person has no idea what the actual costs are in making a book. The cost between making a physical book and an ebook are very close, believe it or not. All the costs that go into the physical book (author royalties, editorial, design, production, sales, marketing, etc) also go into making an ebook. We do save a chunk on print, paper, and bind, but the profit margin on books is so low that it's not a huge difference.
The biggest threat to book publishers right now is Amazon, and that's because they're able to buy and sell books at a loss. Amazon loses money every time they sell a book, they make profit on everything else. So the actual dollar value of a book has been deflated because of their ability to manipulate prices, and publishers literally cannot afford to produce and sell books at that rate, and the people that end up losing money are the authors. There's this huge misconception that publishers are hoarding money and taking it away from authors, but they're really not. Self-publishing has its advantages, but publishers offer a lot as well, all of which costs money. The book market doesn't work the same way as the typical consumer good and that's widely misunderstood.
I can get into this more if anyone is interested. It's pretty interesting.