Just to chime in about tenure: Tenure includes job protections, but it includes no protections against being reassigned to teach in another discipline. Yeah, you wouldn’t reassign a tenured faculty member into something quite distant—that would be silly, not to mention detrimental to the whole enterprise—but into an allied field? Yeah, that happens.
Of course, less than half of all faculty in the United States are tenure-line, anyway—not even counting graduate teaching assistants!—so the claim that the immovability of tenure line folks prevents nimbleness in reacting to enrollment trends is a red herring, anyway.