Purely from an engineering perspective, in a setting where we care about privacy --- ostensibly, email is one of the settings where we care most about privacy --- the ability of a stranger to authenticate any particular historical email is a fairly grave vulnerability. Privacy-preserving systems go out of their way not to have it.

You may have some kind of public policy or archaeological argument in favor of the vulnerability, but then it's hard to see how the same argument doesn't suggest we should compromise the entire confidentiality of every email; after all, that would make research a lot easier.