I mean, if you come back years later and lay a claim to an account you're going to have to show something that proves who you are.

An SMS sent to the phone number stored on the account is no more reliable than asking the user to generate a code with an authenticator app (based on a secret that is stored in both the account and the app). People can lose the app/phone just as easily as the number. Also, SMS confirmations suffer from many vulnerabilities that TOTP codes do not.

The main point is that these methods are not related. Google could and should offer them side by side. Let people take their pick of any of the following:

• Confirmation message sent by email (and let people add multiple address not just one).
• SMS to phone number (again, let them add multiple numbers).
• TOTP code generated with authenticator app.
• One-time-use secret codes written down somewhere.
• Secret question/answer pairs.
• Codes generated by USB key fobs.
• Confirmation on a phone that's still logged in to that Google account (this doesn't require the phone number).

Google is witholding some of these methods until you give them your main phone number, which is obviously a ploy to get your main number so they can track you.

I'm frankly surprised that a privacy-oriented community is not aware of the fact phone numbers are an excellent means of tracking people across services and databases for extended periods of time.