Good point, but if Google suspends my account I've got bigger things to worry about than the dozens of sites I've used once or twice a year. Paying for your own domain also comes with its own troubles. If you're not using Google (or some other service) as your mail forwarder, good luck being able to email anyone. Stealing you custom domain is also a real possibility, and negates your investment in Gmail 2FA.
How could my domain be stolen? :O
- does your registrar send Auth-Info code over email in plain text?
- did you enter real contact and residence data when registering the domain including public WHOIS database?
This is only a fraction of the attack vector.