Increasing costs to add a driver does not prevent company's from adding new drivers it just makes that more expensive. Thus, you need to consider it as another form of cost savings not a difference in the number of drivers.

A company that pays 50$ less to get an employee can reduce fairs on the order of 1 cent or make an extra 1 cent per ride. It's possible Uber might add a few cents per ride this way, but it's not particularly significant even assuming the average driver does not last that long.