CBC is pretty much obsolete. Modern TLS uses CTR based ciphers (AES-GCM or ChaCha-Poly1304). CTR can be more efficient than CBC, because encryption can be parallelized, while CBC has no advantages over CBC for this use-case.
Encryption also would likely change independently of the protocol issues.
Layers of communication are a thing for a reason, it allows each piece of the communication to upgrade independently. Instead of writing encryption into the spec, its more useful to write encryption on top of the spec as a 2nd layer.
