The question shouldn’t be "when will the WP REST API endpoints be committed to core?" but rather "how can the WordPress project ship solutions to more and better Big Hairy Ambitious Goals?"
If we can solve this organizational design challenge, then landing the WP REST API endpoints, and other large improvements to the project, becomes an "easy" task (or at least one requiring a known amount of people with a known amount of involvement). If we can’t solve this organizational design challenge, then it does us no good to commit the endpoints to WordPress core. My biggest fear is for the endpoints to become another media library.
Specific to the WP REST API endpoints, I think it’s time for WordPress to aggressively pursue the component maintainer model. We need effective, reliable component maintainership to take responsibility for producing quality endpoints for each component.