Matt Mullenweg wrote an update on Make WordPress regarding core focus areas. He says writing and customization are coming along but regarding the REST API, notes, “There has been little to no perceivable progress on having any parts of wp-admin powered by the REST API.”
Several of the changes in 4.7.4, however, were in relation to the API. But his point that switching out functionality is proving more challenging than first thought — mostly due to back compat issues — is true.