They do have a history of that, but AppEngine has a huge footprint right now, and Google is only expanding it's cloud platform with Compute Engine, redoing UI, etc. They're clearly investing a lot here, I don't think they'll close this as easily as Google Checkout or others
Free services are not guarnateed to remain open. Paid services can continue to live for a long time until market share shrinks so low. GAE can't. It has a deep root in the market.