The price worth it because its just for one time payment. And threema refuses to add auto deleting messages features because nothing stop your receiver to take a photo before the message disappear there is also many ways to keep incoming logs thus giving a false sense of security that in reality does not exist.

In another way threema can't implement that feature because

after the messages is delivered to your receiver its deleted from threema servers threema as nothing from you nothing to be deleted.

And i full agree with this i need to trust my receiver to send them a message a secure messenger its only to protect messages from third party actors.