ZD, I think you bring up a good point that a large store has more to lose than a small one, but those who don’t drink cholov Stam don’t differentiate between large dairies and small ones.
I think if you’re contemplating how likely Starbucks is to switch the milk, think about how awkward it would be for Starbucks to tell all their customers that they’re out of milk, which is more likely to happen if they’re committed to only use one relatively small, independent brand.