Good point on there needing to be some application-level addressing anyway. On top of that, I believe the UDP checksum can be omitted as well at least on some

Good point on there needing to be some application-level addressing anyway. On top of that, I believe the UDP checksum can be omitted as well at least on some OSes (and is arguably not necessary for fully encrypted/authenticated payloads) – leaving really just the two bytes for the "length field".