>App Engine have actually been around for a long time
That means very little, I hope you realize. Reader, Voice, Chat, etc.[0] were all around a long time.
>Also, there's no inherent lock-in
GAE has plenty of proprietary APIs you can depend on. Whether or not you do is up to the programmer.
0 - A comment below notes that voice and chat aren't deprecated yet. Voice was announced deprecated, and google has had so many chat apps I'm not sure which ones are gone. Anyway, here is a more complete list of things Google has abandoned: https://killedbygoogle.com/
Also, there's no inherent lock-in, you can basically just deploy it somewhere else.
Data is where the lock-in lies. Moving can be hard if you use proprietary databases. Can still be worth it.