One faculty member’s arrival or departure could be more of an issue with small departments, or less common subareas.
However, course offerings and curricula can be changed for other reasons. Better run departments will provide smooth transitions for existing students (e.g. allowing students to graduate fulfilling the old requirements and/or providing appropriate transition courses), but worse run departments may fail at this, resulting in students involuntarily being unable to graduate on time (e.g. SDSU EE department: http://talk.qa.collegeconfidential.com/college-life/2074999-my-school-made-horrible-changes-p1.html ).