Maybe Google is distributed enough to stay up, but I'm pretty sure Microsoft and Amazon are not, and I doubt Google has the spare capacity to handle Azure load, much less Amazon load.
Utilization and availability are not directly related. Much of their utilization is probably not super time critical and could be proportionately shed or suspended in an emergency situation like the one in question.
I bet Google could absorb 100% of the emergency evacuation of AWS in a pinch. Remember also that not 100% of services will be moved, and those that are will not be running at 100% capacity - everyone would be suspending their batch/analytics/warehousing jobs that week (or month).
EDIT: Also, lock in would stop all the redshift/dynamo/etc users from migrating, and would limit people to just what they could throw up on GCE/GAE.