I think a lot of investors and founders try to replicate the model that worked for Paypal. By accepting a high customer acquisition cost (in the form of literally giving away money) at the start you can scale up phenomenally quickly. It's a proven model. The problem is that it's hard to know if you can still attract new customers without that artificial appeal, so actually stopping is a huge risk. Your growth figures could collapse. Uber have managed to amass such a ridiculous warchest of cash that they won't need to stop for a really long time. And I suppose that's the point - if they can get to profit even with the subsidies before they run out of money they won't ever have to stop giving away money to new customers.