The problem lies in that Google Authenticator is tied to a device, so if you upgrade it or lose it, you’re f’d. I also doubt many use/print recovery codes, and if they do, good luck finding them 7 years later.
Overall the situation isn’t great.
Overall the situation isn’t great.
It was a pain for all of them, but it was worst for the ones that I had no other auth systems set up. (Or the ones that had my old phone number for SMS still, even though I thought I'd changed it everywhere.)
In the end, there's still no good system for real security. You're either stuck with a device you might lose (or someone might steal), or stuck with an account that you might cancel (or someone might steal). Or use biometrics which are just not ready for prime time.
You can save the QR code that was used during setup to repeat the onboarding at any time. You can also use Authy, 1Password, or another service that lets you store the one-time password somewhere else. Or use U2F devices when possible.
So the likelihood of moving to a new phone without those codes transferred is very high. Not exactly an easy experience.
When you’re at Google scale, all of these methods have real world flaws.