- A CA validates requests, signs CSRs, publishes cert revocation, issues certificates and trust anchors.
- A registrar in DANE merely passes a DS record you created to the TLD, along with the promise that this record was created by the domain zone owner. It's basically the validation step. Nothing to do with establishing or securing data, key/record management, etc; they're a glorified FTP tool.
I'm in favor of registrars getting more involved (since they are the authority on who controls a domain), but only with a completely different design. I have suggested many times that CAs establish an API to communicate directly with Registrars to perform the validation step, as this would eliminate 95% of attacks on Web PKI without introducing any downsides. So far my pleas have fallen on deaf ears. And since the oligopoly of browser vendors continue their attacks on system reliability (via ridiculous expiration times) without any real pushback, I don't see it changing.