How do you validate any certificate? You'd have to trust the registrar, presumably like you trust any one CA today. The web browsers do a decent job keeping up to date with this and new top domains aren't added on a daily basis anyway.

Utilizing DNS, whois, or a purpose built protocol directly would alleviate the problem altogether but should probably be done by way of an updated TLS specification.

Any realistic migration should probably exist alongside the public CA model for a very long time.