That's actually a good point. Giving people a "minimum guaranteed time" for APIs and services would give a lot more confidence to people looking to build on them, and also keep data portability at top of mind when building on those services.
Some of Google's API do have this, it'll be in their terms of service under "Deprecation Policy". The deprecation policy is a promise that if the service is going to be discontinued you'll have at least X notice.
The examples I can think of off hand are all Google App Engine related but I wouldn't be surprised if some of the Drive APIs have the same thing.
I'm pretty sure they already know the minimum time they want to keep a certain service afloat. Would they really lose much if they publicly announced a date, after which they would reevaluate a service? I think I'm giving Google free PR advice here... damn you Google!
