"What if they can register a very similar / regional domain that you didn’t set up already?"
In other words, they register fakebook.com and then just go get a TLS cert for it. If you're not looking carefully, you might not notice the difference.
Whether the CA system, with fungible, interchangeable certificates that can be issued by dozens of CA's (pinning excepted), is worth sinking lots of trust into is an entirely different matter ;)