However, in practice, using "Sign in with Google" by default dramatically increases resilience to most failure modes for almost everyone without a sophisticated threat model.
> To save what? An additional password?
The average person does not manage their passwords in a sophisticated way; if someone is signing up for many different services, they are probably using the same password everywhere, or some simple enumeration scheme. Then, when some random forum or web service gets compromised (as they inevitably are), their password to everything is compromised - including Google! On the opposite end of the spectrum, if Google is the only service with a password, and everything else is driven by Google-owned SSO, that person is essentially immune to compromise; whatever I think about the company, I do believe in Google's security team to keep passwords safe and block dictionary attacks more than any other service on the internet.
And while we can evangelize setting up password managers and using strong random passwords everywhere, the truth of the matter is that many people simply cannot accomplish this. "Sign in with {Google, Facebook, Microsoft}" gets them 95% of the benefit at much higher reliability.