Google Fiber, like Android, is a moonshot to keep their line of business alive. Failure doesn't mean that Google doesn't profit off a new line of business. Failure means that Google can be shut down.
Google lives off open internet. Their profit is primarily off:
1. Android 2. Search 3. AdSense, AdWords.
A closed internet means that 2 out of 3 is dead. All they have in Android.
If AT&T and Cable monopolize the internet (which they are close to doing) without strong net-neutrality laws, they can (and will) slowly migrate to the internet being limited to the top 100 sites (curated by them).
In such an internet, search and AdWords/AdSense is useless.