It would seem that the "secrets roach motel" model was the most secure and sane option for Authenticator. In that way, it mimicks the Yubikey or other hardware TOTP keys, where the secrets go in but they can't come out.

Everyone with a Yubikey knows that the secrets aren't coming out and they will need a backup. It would be foolhardy to rely on a Yubikey alone, when the risk model obviously entails loss or theft of the device itself. That's what paper codes are for.

It would seem that the QR export, and now this account sync, is Google weakening security as a concession to intense end-user pressure.

Yeah, it sucks to lose all your TOTP secrets at once; that's why you form a contingency plan and stow your paper backup or maintain a spare device.