20-Year Prospect
We can certainly say that rare books will be around in twenty years, and there'll even be people looking to buy them. Beyond that, though, it's difficult to predict how exactly the market will look.
If current trends continue, it's likely that rare book selling will be dominated by a few big companies that have the resources to buy big-ticket books and maintain a strong web presence. If there's a spot for the independent bookseller in that future, it won't be a big one.
That said, you never know what can happen in retail these days. Maybe the rise of e-books will set off a collector's craze for all the old cloth-and-binding books, expanding the market for higher-priced collectible items.
The internet could actually be the perfect catalyst for the rare book industry. If you have your sights set on rare book dealing, just make sure to learn your stuff, keep up with online retail trends, and hope against hope that people will still be paying top-dollar for old tomes in twenty years.