If you recall that the NSA is already storing large amounts of internet traffic for later decryption, whether or not some server stores it is mostly irrelevant.

If you recall that the NSA is already storing large amounts of internet traffic for later decryption, whether or not some server stores it is mostly irrelevant. Maybe you're worried about other lower-tech attackers, but I don't think that Matrix servers are going to keep 30 years of chat history (I imagine most Matrix servers are going to eventually use default retention policies). Not to mention you can run your own homeserver if you don't trust the operators of public homeservers. Storing 1:1 messages (noting that Matrix doesn't differentiate between group chats and 1:1 chats) on a server is beneficial because you might not want to lose your entire private chat history with your family and friends whenever you lose a device (source: this happened to me multiple times with Signal -- which has an awful backup system that I didn't use because it simply doesn't work properly). Now, there is obviously cases where you don't want the server to keep your messages forever -- and then you might want to have retention policies and so on. But I don't really agree with the security argument, since the most obvious attackers already likely have your messages, and it sacrifices a lot of convenience that most people expect (my partner didn't expect to lose all her Signal messages when her phone died unexpectedly).