A motor carrier of passengers may discontinue its entire service on any route upon 30 days’ published notice to be prescribed by the department, and thereupon its certificate therefor shall be canceled. A motor 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 department 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.