Apple already admitted years back that it aggregates anonymized location data from iOS devices. It started doing so long before the release of iOS 6. The point being, even if you use a different app for directions, Apple can still presumably gather useful data about traffic patterns, Wi-Fi hotspot locations, and other valuable location-based data.
And Apple's Maps app is pretty explicitly not monetized. There are no ads. So Apple isn't losing out on lucrative advertising revenue when users turn to Google Maps instead. Since Apple makes its money primarily from the sale of its devices, rather than by ads or selling other content, it's already acquired your cash when you bought your iPhone.
But Apple does lose something with the arrival of Google Maps. Specifically, it sheds the increasingly bad reputation that, rightly or not, its own mapping solution has earned since its release. Beginning and power users alike understand the App Store, and anyone with a compatible iPhone can figure out how to find, install, and use Google's Maps app. That takes a lot of focus off Apple's own Maps app, and gives the company some breathing room to improve its own offering.
What does Apple do next?
Google's long been an active, clever App Store developer. There's no doubt that the search giant will continue to improve and refine Google Maps. Eventual iPad compatibility seems like a sure thing, as well as likely integration with your iOS contacts (so you can quickly navigate to their addresses), and perhaps Google Now-style speech support for requesting directions. Given the way Google has building its own little microcosm inside Apple's apps, it seems probable that you'll be able to request Google Maps directions from Google's Search app and Google Chrome.
But what about Apple? If, in the end, the company got just what it wanted -- a robust, full-featured Maps app, albeit provided by Google as a third-party app -- you could argue that Apple could quietly let its Maps app follow the Newton, OpenDoc, MobileMe, and Ping into the Great Retired Apple Offering Garden in the heavens, high above the iClouds.
That won't happen. Apple can't and won't allow a core feature as important as maps to be owned and defined by a third party, no matter how much work and struggle might remain for Cupertino. Though it's exceedingly unlikely, Google could one day pull the plug on its own app. More realistically, Google Maps could get saddled with ads or mandatory Google Plus integration or other features that would drive Apple crazy.
Thus, there's only one sane, shrewd path for Apple to take: Welcome the much-needed reprieve offered by Google Maps for iPhone's arrival, and keep on working behind the scenes to make its own mapping data ever more competitive.