1. I'm not sure there's any evidence of Apple loosening its grip the app store because a certain genre is popular on Android (eg Swype)

2. Apps take longer to develop, require more skill & time investment, and (if iOS) have to go through byzantine approval process. This mean longer iterations, less risk taking => slower innovation

3. The higher barrier of entry keeps a lot of people of the type that made the Web interesting from creating apps. Web dev was simple enough to allow non-techies to create some pretty interesting stuff. Most apps feel like they've been created by corporate dev teams or geeky college kids.

4. There's definitely a class of apps which couldn't be implemented as web apps - but that's mostly because the OS provides a limited API to the phone's features, not some inherent weakness of the Web model.