I'm not necessarily a fan of Thieve's Cant, but I can't understand why everyone is this upset over it. Thieve's Cant is as easy as anything else to refluff, in some way, you are familiar with slang and doublespeak. It's still relatively wide open. Now, I'd be fine if they slipped it into the thief background, in fact, that might still be a better option(or make it a language anyone can learn), but I don't think they need to.