Google has been making significant visible investments in GAE, even with the advent of Compute Engine. There is also a hybridization of the two going on. While I can't claim to expect my code to live unaltered forever, I'm not worried about having an upgrade path. Also: I do have a lot of GAE apps that have been running nearly unaltered for several years. So there is a good track record.

For a point of reference, when Google rolled out the High Replication Datastore, they kept around the Master/Slave datastore for years and even built a pushbutton tool to automatically migrate apps. GAE has been around since 2008; this is not an experiment like Wave.