> what would happen if I were to lose the domain name or server

On the other side of the coin, what would happen if google bans you? That's certainly not unheard of.

True, something else I've considered. But the reported incidents are few and far between, and the stories always have some trigger. I have been taking small steps to move away, though it's yet another service where "stuff" could happen: since DuckDuckGo introduced their proxy email service I got one and have been using it for my more recent sign-ups. So if something does happen to the GMail or I hit some switch-over threshold, I can point it elsewhere, though everything before would be lost.

If Google bans you they at least will not hand out your email address to someone else. If your domain expires and someone else registers it that’s essentially what’s going to happen.