I would be willing to bet that a majority of the people that see a TV show or movie that shows someone drinking coffee from one of these cups, will immediately know it is from Starbucks, even if just a portion of the cup is displayed.
You don't need to see the entire logo to know who is it. That distinctive green sets it apart. To me, that is successful branding.
So, why would they change from the green to a brown logo? Anyone?