How many Starbucks customers buy just coffee?

This is shown to be a hit in European cities, where just based on a guess espresso and coffee is more popular. The majority of Starbucks customers are probably buying mixed specialty drinks.

Watching a Sbux barista make just a few drink, they probably go through a gallon jug of milk in minutes at peak times. How much fresh chilled milk can this cart hold? Not saying there's no market, but taking on Sbux seems a bit mis-targeted.