> Trusting Google's OAuth not to vanish overnight

Sorry if I wasn't clear. It is not that google will remove the service overnight (although they are infamous for canceling things, but not that bad). The problem is google will lock out users randomly for no reason and no recourse.

If that user was using google login to access your service/tool, you lost that user and there is nothing you can do. You really don't want to gate the access to your product via an unreachable unresponsive third party like google.

Many well-established web frameworks have plugins or components to handle user management out of the box, with sane defaults. Nobody should have to roll them by themselves with each hobby project. You're probably using a similar plugin to integrate with Google anyway.