I've been teaching in universities for over thirty years, Mark, and I've known very few professors who "weren't worth a damn after tenure." If you're not an academic yourself, I'm inclined to question the breadth of relevance of your experience.

Every university evaluates its faculty differently, but at mine, all faculty, from adjuncts to full professors, are evaluated by all students in every class. (Students can opt out, and because of survey fatigue, many do.) Department chairs write an evaluation of every member of their department, and the evaluation includes a category "Needs Improvement." Even tenured faculty who receive that designation have to work out a plan for improvement with their chair and dean. It would take years of failures for a professor to be let go, true, but I'm not sure that the "performance review" that you imagine would solve the problem, either. What do you measure? Teaching? Do the students evaluate it or another professor, since we'd want professors to teach new classes, innovate, and sometimes fail? Publications? Which ones: online publications, or just books and articles? (Books take years to appear in print.) University service? All sorts or just some? Do you award points? Is there a minimum that faculty have to reach for renewal? Is there an appeal process?

Again, if you're an academic, I apologize for telling you what you already know. But I suspect that you are not, given the argument you make, which seems uninformed by actual university practices.

Thanks for reading and responding, my friend, and all best to you.