If Uber has a (simplified) split cost per drive consisting of vehicle (+maintenance) and/or fuel & driver. If subsidised part is generally covering the driver part. When you replace driver with autonomous vehicle and you remove subsidies, you're left with a sustainable (presumably) model on a certain margin that is already rolling. Rolling in a sense that it is already an established business - people know it and use it. You've used subsidies (well, investors cash) to build a business.
Of course, this relies on a presumption they will build a sustainable model on replacing drivers with autonomous vehicles. It also presumes they will not venture into other, (potentially) more profitable business like logistics.
One thing is certain. They are positioning themselves for a great catch which relies on few key components working in the (near) future.
I think real hazard for Uber is regulation (autonomous vehicles for example) and market regulations (see taxi debates in Europe).