In reply to Christoph.

WordPress has not traditionally shipped major features as opt-in. You don't get the usage and feedback you need by hiding it away.

The template editor is mature and ready to ship. The problems I covered above are theme issues, and you're always going to have those issues until themes catch up. Making it opt-in will only slow that growth. It's been the same with nearly every front-end feature that's come to WordPress over the years. Core ships something and theme authors add in support for it.