Tenure expiration date?

Tenure: Mend It, Don't End It  Arthur Levine describes some the advantages and - in no uncertain terms - gives his views on the weaknesses of the tenure system.

More often tenure provides a lifetime of job security not to professors whose work requires protection, but to a significant minority of "deadwood" — individuals who are unproductive, out of date, or poor in their research, teaching or institutional commitment.

His suggestion is to have a 30 year contract to limit the length of tenure and thus insure an institution's ability to maintain 'intellectual vitality'.