It's entirely possible we will never need to replace AES (or indeed ChaCha20). AES 128 GCM is mandatory to implement, and implementers are recommended to offer ChaCha20 too. If you don't have AES hardware, ChaCha20 looks very attractive and you'd be a fool not to.
So while you can get code points for a vanity cipher, there not only isn't an appetite from the working group to endorse such ciphers, there is unlikely to ever be a scenario where a client or server would actually pick it unless you control both and choose to prefer the vanity cipher. Whereupon why bother?
The downgrade prevention in TLS 1.3 is pretty effective, so an attack which needs a downgrade also needs to defeat the downgrade protection, an additional hurdle. Obviously people should get rid of TLS 1.0 and TLS 1.1 but meanwhile I think the anti-downgrade story is the best it has ever been.
