I don´t think Taylor is very good at math, but i guess she's not completely stupid, she probably leaves that to other people. In fact, when not finding her music on spotify, her fans will turn to other sources to get her music, probably record stores or itunes, it's not that people that go into spotify buy anything, they buy what they like, and if it's not there, they will look for other means to get it. So i'm not that sure that she will loose all those millions you mention... She just has to be very on top of the pirate sites so that her fans have no option but to buy her stuff from itunes or record stores.