This really miscomprehends what forward secrecy means. PFS prevents an adv who obtains the keys from decoding previous messages -- even with access to your unlocked phone (and the long lived keys) they couldn't obtain cleartext on any message you had deleted, even with the ciphertext. Also, having a plaintext message does not confirm it was a particular ciphertext.

But it really isn't available to the software author to know how long we want to keep a message for. If I want I can set a disappearing message timer.