No one is auto-disappearing messages. You are just not understanding.

Evidence that used to exist no longer exists as hard evidence. It isn't "hidden"... everything is still there. With secure DKIM justice can be served. With public after the fact DKIM, a shady lawyer claims the message could have been forged. Would-be hard evidence no longer exists as it can be painted as possibly forged.

Destroy: transitive verb: to put out of existence.

Are you just not learning what words mean?