The usefulness of the service is dependent on the volume. If a 2x price increase cut the volume of passengers dramatically, the volume of drivers would correspondingly fall, and the likelihood of pushing a button on your phone to summon a driver already in your neighborhood likewise falls proportionally. Perhaps to the point where the service is literally valueless.

None of this is to argue against the sustainability of the model, just to point out that no one has any idea what would happen. Network effects matter.