If your app is that big, someone is always carrying a pager for when there are problems. The difference is on PaaS, you can't do a damn thing about it if it's a problem with the platform.

I've helped multiple companies get off of app engine because even for companies losing money (startups), it's too unreliable -- and actually very slow (datastore) if your app is relational. Also, it's very very expensive if you hit the datastore hard.