And they will likely be the first customers, until volumes ramp up and prices go down. There are many places where taxis do not make sense or aren't optimal. Low density areas, daily commute at the peak hour in big cities (or in general any place where you have a one way flow), people using their car professionally, etc.

Using them as taxis makes the initial cost of the sensors less important, not more important, since it's a fixed cost and taxis are driven much more than regular cars (especially true for ones that don't depend on a human driver).