Sure, not everything can be a web app, but a lot of the subscription world can easily be ported to web app, since the content will likely be delivered in HTML.
Interestingly, serving e-books over HTML would destroy the DRM models that Amazon and others have developed to "protect" their content. I can't see there being a Kindle web-based reader unless Amazon takes the same road as Apple with regard to DRM [1], and tells all their content providers to get on board.
If you're talking desktop, I agree, since the browser has become so pre-eminent there. But for mobile, Web apps are distinctly second-tier citizens. Just installing one on your home screen is voodoo beyond the ken of most iPhone users.
It's not so bad on Android. And Google is very good at Web Apps. Try viewing the web app versions of gmail and such on your phone. They're not as full featured as the dedicated apps, but they work damn well. The big thing is making them look like real apps to the end user. No one wants to have to remember a bunch of web addresses. Something like what Chrome does on the PC is good...instead of a classic link shortcut for your desktop, it makes an "app" out of it. Giving it its own icon and name like any other program on the computer.
If that's all the value you're offering your users, DRM is the least of your problems. If it's that easy for users to exhaust the content they actually want, you're going to have problems with customer retention, not to mention customer acquisition.
