I routinely take app engine apps and run them on the FOSS AppScale platform unchanged. For the things you can't port, like using Google as your auth, there are

I routinely take app engine apps and run them on the FOSS AppScale platform unchanged. For the things you can't port, like using Google as your auth, there are 3rd party solutions for that already. Choosing Google's App Engine is more than just choosing a managed solution, too. You choose it because of Google's reach: consider, for a moment, how you send a push notification to an Android device. When you do it from Google's cloud, your metal is co-located with their push infra. Also caching, logging, tracing and even a global CDN is integrated into the platform. Think of the cost of having to sort this yourself.