The idea is that the cost of autonomous car use will be much lower than taxis, because there are no labor costs. If you have to get to work every day, taking a taxi (if you can even find one) is much more expensive than buying a car and amortizing the cost across its lifetime. As the price of autonomous taxis fall, that will reverse.

That doesn't mean I agree with the GP's point about it lowering car usage overall. The reduced cost of auto taxies also pushes against your reluctance to take one, though perhaps not all the way to "use my car whenever I leave the house" levels. I also think that once people begin replacing their cars with autonomous taxis, they'll sign up for all kinds of taxi subscriptions that will further reduce that reluctance. After all, driving your car now isn't completely free: it still costs you gas money, plus the hassle of actually driving it. And other forms of transportation aren't free either. So the bar here isn't 0.