A motor common or contract carrier of passengers may discontinue its entire service on any route upon 30 days' published notice to be prescribed by the commissioner, and thereupon its certificate therefor shall be canceled. A motor common or contract carrier of passengers may discontinue any part of its service on any route upon 30 days' published notice, subject, however, to the right of the commissioner to withdraw its certificate for such route if, in the opinion of the commissioner, such diminished service is not adequate or is no longer compatible with the public interest.