I may be wrong but it looks like Google's fear of the mobile ecosystem - that apps cannot be crawled, indexed and searched for may be an overreaction. It is precisely because apps cannot be crawled, indexed and searched for, the mobile application ecosystem's expansion will be limited - mobile app discovery will continue to rely on the web. Apps are isolated and cannot link to a particular state of another app. No links, no network effects. The expansion of the app ecosystem is currently limited by the number of apps in the market; With each new app in the market users will require more time to search for a suitable app. More websites, however, actually encourages even more websites, due to linking and references between all the websites; More websites enable users to discover faster the website they derive the most value out of (due to linking). More mobile applications, on the other hand, do not. It's almost like binary tree search O(log(n)) vs iterative search O(n).
These are just my opinions, however, so take it with a grain of salt.