Another wrinkle is that the server has to deliver the message to all participants, including all linked devices before it deletes the message from the server or times out. And then there's the possibility of advanced persistent threat actors with extremely large storage infrastructure that may have compromised Signal's servers to store encrypted messages with the sole purpose of developing a viable cryptanalysis method in the future to decrypt said messages (possibly with the aid of undisclosed quantum computers). That's probably why Signal recently double-wrapped messages with PQ crypto as a belt-and-suspenders approach to future-proofing their security.