There's nothing self-destructing about your server dropping data, after a decision on (unencrypted) meta data (timeout, number of views).
Server sees expiry, view count, salt, iv. Content is encrypted, metadata isn't. Can't avoid this with server-managed TTL - alternative is client-only expiry but then you're trusting the recipient's browser.
Main point is credentials don't live forever in chat history. Smaller window, not magic.