Because there is no one left to win over or impress (the front half of the bell curve has been well saturated), Starbucks' wave is past its crest. They need to shake up their elite beverage world.
Tall, Grande, Venti - Magno! They should introduce a new size that mirrors the excess their customers perceive as a need. Just two cents worth of advice to a gadzillion dollar marketing dynasty. You heard it here first.