People have had their Google accounts suspended (especially if it's associated to say, a YouTube or AdWords account), and now you have no access to your address anymore, and Googles free tier support won't help you.

With domains there's an ICANN process to get your domain back if it gets hijacked.

(a) Using your own domain doesn't require using your own mail server, you can point your mx server at Google apps if you're comfortable or your registrar probably supports mail forward if you don't want to pay for G Suite. (B) it means you can keep your email address if you ever leave Gmail.

I understand that, but there's still the possibility of the domain name itself being hijacked, or even just forgetting to renew it.

Maybe I'm paranoid :)