Maybe so, but I'm struggling to imagine a scenario where that comes true. As we generally think of them now, they're for the suburban commuter to replace their current car. As such, they can't be limited much, lest only a tiny elite be able to purchase them- probably not tenable politically. Furthermore, politicians don't seem farsighted enough to even set quotas if they wanted to; any limits set would likely be at something resembling estimated real-world demand, and since we're talking about a shift that would radically reduce the total number of cars in service, politicians would likely overestimate the need.

Plus, driverless cars can't form taxi associations with reasonably large voter blocs.