It took four years to change the titles of 30 cities, and you don't think it would be disruptive to change tens of thousands? And worse yet, to argue about "primary meaning" and "necessary disambiguation" on thousands of talk pages? The "good reason" that is needed is a clear, non-arguable guideline, and that is what we have now. To make every single city a judgment call - what an enormous waste of Wikipedia time! Avoiding THAT situation is a "good reason". Consistency for the sake of consistency is not a "good reason".