>It's not negated because a passive attacker that records communications and then, in the (potentially far) future, somehow can obtain the key (say, by exploiti

>It's not negated because a passive attacker that records communications and then, in the (potentially far) future, somehow can obtain the key (say, by exploiting some weakness and/or brute-forcing), still cannot decrypt your past communications, regardless of whether everybody saves old messages or not. If someone breaks the encryption somehow then forward secrecy is also negated. They get the encrypted material directly. Forward secrecy is only effective in messaging if the attacker does something like break into your device to get the secret key material. At that point they will also get any saved messages that are still accessible to you, in whatever way they are accessible. Now in theory a messaging client could reencrypt old messages to something like a public key pair where the secret key material was protected by a strong passphrase. But no one does that because that would mean you would have to type in the passphrase whenever you wanted to see an old message. At that point you might as well just use encrypted email and leave everything encrypted all the time.