The difference is that with e2e encryption the carriers don't see your plaintext, while they do with TLS since they terminate the TLS connection on their end.
If RCS is a standard what's stopping someone from writing an app that encrypts your plaintext? Sure, it'll leak meta data, but at least the actual plaintext is encrypted.
Couldn’t you just do the same thing with sms today?
Already done ~10 years ago, it was TextSecure, the predecessor to Signal.
Nothing, but you need all your contacts to use a compatible client. I really hope something like https://messaginglayersecurity.rocks/ takes off and that we end up with good e2e interop without having to rely on a single vendor.