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I'd say on average anyone who is concerned enough about their security to turn on an optional 2FA feature is also going to have 2FA enabled on their email as well. If I were only allowed to have 2FA enabled on a single account, it would be a very difficult decision between my password manager and my email. As you point out, email access is the key to the kingdom. Also, the only password that I do not put in my password manager is my email account password.