Why does it guarantee insecurity? Clients have to store events privately already (many use hardware-kept keys), so I don't think the extra encryption from MLS payloads will make a difference. Plus, chat screens tend to require lots of secondary cache/storage to make sure screens load fast, like the last message of each person to build the chat's home screen, pre-parsing markdown/quotes into their own cache, etc. There is no way the protocol can protect from a lousy client.