> Recovery mechanisms for private key based identities have been designed and implemented.
That is not how blockchain works. Once someone has your private key they can make any transactions that they want, you cannot undo those changes once they are on the blockchain.
A blockchain can be used to tie additional information to the public key.
> The site you're actually trying to use could switch to a different blockchain (which is more likely than only offering login with Google or that being canceled).
I hope they don't use any blockchain at all.
> Unlike the Google scenario, there's no way to regain control of your account
Recovery mechanisms for private key based identities have been designed and implemented.