Open Is Better Than Nothing
If a company forks Android, the OS will already be compatible with millions of apps; a company just needs to build its own app store and get everything uploaded. In theory, you'd have a non-Google OS with a ton of apps, virtually overnight. If a company other than Google can come up with a way to make Android better than it is now, it would be able to build a serious competitor and possibly threaten Google's smartphone dominance. This is the biggest danger to Google's current position: a successful, alternative Android distribution.
And a few companies are taking a swing at separating Google from Android. The most successful, high-profile alternative version of Android is Amazon's Kindle Fire. Amazon takes AOSP, skips all the usual Google add-ons, and provides its own app store, content stores, browser, cloud storage, and e-mail. The entire country of China skips the Google part of Android, too. Most Google services are banned, so the only option there is an alternate version. In both of these cases, Google's Android code is used, and it gets nothing for it.
It's easy to give something away when you're in last place with zero marketshare, precisely where Android started. When you're in first place though, it's a little harder to be so open and welcoming.
"Don't be evil" stops where winning starts. Not that this is actually evil, mind you -- but it's still a far cry from Google's promise of "open."