I was wondering... Maybe I don't get the point here, don't know. I think that the rewrite api is still usefull for creating beautiful frontend urls for regular users. But I'm not sure on adding a json endpoint via ep_masks (for example: https://make.wordpress.org/plugins/2012/06/07/rewrite-endpoints-api/).
Isn'it a thing already covered by the new rest api? Am I missing something?
And by the way... Time to create a 'rest-api' tag here in http://wordpress.stackexchange.com?