While I agree with most of your post, I disagree with the last sentence.
I don't think you can count the "cost of Google walking away from Android" unless you also count the "cost of Microsoft walking away from WP7", unless you think there's a significant risk differential between the two events (which you have not justified in your post).
In fact, given that Android is open source, the "shit-hits-the-fan because we bet our entire company on the stack" risk-cost for Android is _lower_ because if the penalty was in fact that high, you could continue development in house.
And you can't argue that Microsoft is more vested in the WP7 stack than Android, because
1. Android has multiple users and manufacturers who would collaborate on the source if Google abandoned it.
2. Microsoft has walked away from platforms that sucked in the past (Zune).
There is a significant risk differential and your post points it out. Google can walk away from Android more easily because their core IP is not present in the codebase and therefore providing it to the open source community does not create competitors to their primary revenue stream. WP7 on the other hand, has a code base which almost certainly incorporates significant trade secrets upon which their core business (operating systems) relies.
In addition, Microsoft has not walked away from Zune - they have simply abandoned the manufacture of proprietary hardware under the brand. Zune as software is alive and well (and a component of WP7).
I don't think you can count the "cost of Google walking away from Android" unless you also count the "cost of Microsoft walking away from WP7", unless you think there's a significant risk differential between the two events (which you have not justified in your post).
In fact, given that Android is open source, the "shit-hits-the-fan because we bet our entire company on the stack" risk-cost for Android is _lower_ because if the penalty was in fact that high, you could continue development in house.
And you can't argue that Microsoft is more vested in the WP7 stack than Android, because
1. Android has multiple users and manufacturers who would collaborate on the source if Google abandoned it. 2. Microsoft has walked away from platforms that sucked in the past (Zune).