Member of the App Engine team here. In the context of the second-generation runtimes, this is one of the big reasons we're pushing people to use standard libraries and public cloud services rather than App Engine specific services accessed through syscall magic that doesn't work elsewhere. We want you to be able to run your app locally or on a non-Google server (or on Flex, GCE, GKE, etc).

You should really consider either very detailed porting guides or compatibility libraries, at least from what I hear those required changes will keep some projects on first-gen otherwise (and thus Python 2, and thus requests to dependencies to please keep Python 2 supported and ...)