I personally doubt that the Walking Dead is driving that many more people to the event that wouldn't be coming anyways at this point. I can't see how there wouldn't be a diminishing returns effect from having it year after year. I suppose the only question is, is how many years will it take before the profit earned by the ticket sales, merchandise, food/drink purchases, etc., from the number of extra people they feel it drives to the event diminishes to the point where it's not worth the cost and effort of licensing and implementing the IP.