I do not believe a manager's competency for the job - both in football and other industries - should simply be made on the basis of longevity, as you seem to be implying. It should be on the basis of results achieved from resources made available to them. And when you look at both the recent Liverpool managers (Rogers, King Kenny et al.) results and Wenger's results at Arsenal, none have performed at an optimum level, given the resources available, or in Liverpool's case expended.
Just as a little suggestionette , but it is always helpful when making a case for something to also get your facts right. Wenger has been at Arsenal for almost 20 years. :