They're already the cheapest if they match prices because there is no tip required. In the long run they'll inevitably be cheaper because of higher utilization of their vehicles and (vastly) lower labor costs.
This is a misunderstanding of the economics of taxi firms. Taxi companies don't pay for purchase or depreciation of their cars (the drivers do), but Waymo does.
There are no drivers with Waymo. Believe it or not, a living annual wage for a person (or close to it) is vastly more expensive than the annual maintenance and depreciation on even a very expensive vehicle.