Keep in mind that the number of taxis on the road - and thus the number of jobs - is limited in many cities to the number of "medallions" issued (which represents the number of taxis that can be operating in the city at the same time.)
In major cities like SF and NY, the number of drivers is almost certainly actually increasing, as Uber has basically rendered the medallion (and the protection it provides) somewhere near useless for many classes of fares. Some jobs are certainly being moved from black car and taxi companies who can't compete with Uber on overhead, but the taxi market has been highly regulated to the point of limiting job creation for many years.
Uber is a great service - head and shoulders above the entrenched taxi industry. Customers like them better, the drivers like them better; but they are lucky that Goldman Sachs had never created a medallion investment vehicle. If upper middle class doctors and lawyers had seen a drop in investment return you know Uber's "stealing" from the medallion holders would have been stopped.
"In Seattle the number of arrests for DUI to decrease by more than 10 percent." They better be careful here. If they start cutting significantly into state regulators revenue stream they will be stopped.
Uber's best bet is a "negotiated settlement" with local and state authorities to kick-back some of the cash they are diverting.
In major cities like SF and NY, the number of drivers is almost certainly actually increasing, as Uber has basically rendered the medallion (and the protection it provides) somewhere near useless for many classes of fares. Some jobs are certainly being moved from black car and taxi companies who can't compete with Uber on overhead, but the taxi market has been highly regulated to the point of limiting job creation for many years.