Faculty rank and the level of compensation is and must be independent of tenure. Therefore, what is the purpose of tenure? Tenure has increasingly created a false sense of long-term permanent employment notwithstanding performance. Because there are established policies that institutions use to evaluate faculty (including but not limited to post-tenure review which is a much more recent evaluative process and of questionable rigor) that are associated with faculty promotion, ethics, productivity and professional conduct, tenure has over reached its original intent of protection in the classroom and laboratory. Tenure has outlived its usefulness.
Remember that tenure creates a lifetime employment contract between the university and the professor.