I think you're right that Google are worried about users moving to a market where they're not relevant. That's why Google have made a point of becoming relevant: they have a mobile OS, a mobile app market, and a mobile app ad platform.
If people move away from web and toward Google's mobile properties, then they're probably pretty okay with that.
If they move toward Google's mobile properties I would expect they're okay with that. I think Google loses a degree of control over the user once they enter the app even if they are able to advertise. They can't require app makers to put ads into critical locations in the app but they can certainly do that in their search results. Google also doesn't have quite the monopoly over the app market that they do over search.
True. This is probably one of the big reasons that they're trying to make native apps more spiderable by search engines: they want Google Search to continue to be the Internet's entry point, regardless of what app you're using afterwards. That hadn't clicked with me yet - interesting!
If people move away from web and toward Google's mobile properties, then they're probably pretty okay with that.