True that Google stills uses it internally, but external usage exists way beyond Snapchat (although they are a fun customer). App Engine hasn't been in maintenance mode, although we have set aside engineering cycles to improve the reliability and stability of the service in order to ensure the highest level of support for production-grade customers. In tandem, we've some large features such Go, PHP, full-text search, and Cloud Storage support. We've added integration with the Cloud Console as well as Cloud SDK to allow developers to build hybrid solutions that span, for example, App Engine and Compute Engine. And, yes, we've invested in the App Engine Managed VM environment with a host of new runtimes including Java 8, Python 3, Go, PHP, and Node.js. We have more coming there over the next 2-3 months. Guido leaving was tough, I used to share an office with him. That said, I'm not sure that his exit was the writing on the wall.

Disclaimer: I'm the lead for the App Engine product team.