To all the commenters who are complaining about item no. 6, remote message deletion: Yes, of course you cannot be sure that the message is actually deleted. But

To all the commenters who are complaining about item no. 6, remote message deletion: Yes, of course you cannot be sure that the message is actually deleted. But in my case I usually trust the receivers not to be evil, but I do not trust them to be exceedingly careful or infallible. Therefore said feature would protect messages in case their phone gets lost at some point in the future later than the expiry, or in case some third party gets access to their phone in any other way.