But the idea of perfect forward secrecy is not that your locally stored mail is securely encrypted in the future, but instead that mail that was intercepted in transfer should not become readable, even if your key is later recovered.

With your locally saved mail/messages it is up to you how to and whether to securely store them. You can save them decrypted if you are not worried about getting your device stolen. You can securely delete them if you think that you can not keep their content safe. You can do anything in between, it's your choice to make.