Actually we go for the coffee. But there is something to be said for the store experience of Starbucks, it is high-class and delightful, compared to something like Dunkin Donuts.

there is better and cheaper coffee elsewhere, even McDonalds

People who can't taste the difference between McDonalds' coffee and Starbucks, are not in Starbucks demographic. For the price, you can't get a better cup of coffee, unless you're willing to roast the beans yourself.

With that said, the key issues with Starbucks, are long lines, high prices and the wi-fi squatters. I seriously doubt that a cart based coffee stand can address all of these issues.