You're vastly overestimating the utility of the data. Information about the behavior of individual users will be of limited use since Google will mostly not be able to associate an iPhone user to a Google account (unless they actively force users of the app to sign in). And even then the such tracking would really need an opt-in feature to avoid being extremely creepy, and for it to be opt-in it'd need to give users a real benefit. On Android they can do it (e.g. Google Now), it's less clear that it's possible on the iPhone.
There's some value in the aggregate information from all users in improving the service, but the law of diminishing returns will be reached quickly. Losing even half the mobile maps usage would have basically no impact in the ability of Google to deduce things from the data. There might be a few exceptions like real-time traffic information where every data point is precious.
I don't think I'm truly overestimating the utility of the data so much as eliding over the massive engineering and labor required to extract useful information out of it. But that is probably Google's core competency.
I don't think Google's going to get very evil, at least not overnight. They have the infrastructure to collect all this data, the resources to store it, and the technology to make sense of it. They're already at the point where their business regularly butts against social norms regarding privacy.
Ultimately I find advertising an extremely unpleasant (if not inherently evil) phenomenon. At the highest levels it's nearly the science of manipulating unwilling people. Advertising companies, especially nebulous yet megalomaniacal tech companies with access to the best behavioral datasets possible, should be treated as highly suspect.
There's some value in the aggregate information from all users in improving the service, but the law of diminishing returns will be reached quickly. Losing even half the mobile maps usage would have basically no impact in the ability of Google to deduce things from the data. There might be a few exceptions like real-time traffic information where every data point is precious.