It's moving the other way that will get you.

And that will likely wipe out any savings and then some.

Moving to GCP takes a bit of an idealogical shift, but moving off Google is pretty simple if you use AppScale. That might be the best option for these startups; take advantages of the free services from Google and move away after a year if you need to.

AppScale is an alternative implementation of AppEngine, but AppEngine isn't the whole of Google Cloud Platform.