They are working on the UX side of things. I disagree that it's a trivial gain. Signal works in the same way, except that they only really support having a sing

They are working on the UX side of things. I disagree that it's a trivial gain. Signal works in the same way, except that they only really support having a single device (that's where the safety number changing warning comes from). Having per-device keys also means that the server cannot decrypt your messages (because the identity key is not used for communication, it's just used to register new keys that clients will encrypt their messages to). Having a global decryption key stored on the server would be a less secure design.