Well, continuing to use Gmail as an example, there is an account recovery system, which IIRC asks for a bunch of details to try and determine if you are the account owner (account creation date, names of labels used, etc.) If Google or a third party would provide a list of these details, then you could collate that info as additional insurance against your posited scenario.
At some point, there has to be a way to get back into your account. Probably, going through slow and hard to hack methods like the postal system.