Here's a couple more ideas to throw in the pot:
1. The price of ebooks is less than pbooks, so trend comparisons ideally should be in units rather than $$.
2. There has been a massive migration to reading electronically in the last decade, but most of it is not in the form of 'books' as such. It's news sites, emails, blogs, facebook, myspace, forums, reference sites and so on. This overall trend won't stop any time soon, and books will follow it. The devices we use now will look crude in 5 or 10 years time. Almost everybody sneered at mobile phones when they came out, now they're everywhere.
3. Pbooks probably won't die out. After all, many of us still use a pen and paper, and some folks still play harpsichords and other acoustic instruments. But as the E-way of doing things continues the older ways will diminish in market share, and probably change their form as well. There are still blacksmiths who can make you a wrought iron gate, or even a horseshoe - but they're a lot harder to find than when I was a kid, and what they sell to clients has changed.
Cheers,
Chris