There are certainly scenarios where the cost delta between our hypothetical fully-autonomous vehicle and a taxi as we know it today is greater and where it is less.

I'd argue that in Manhattan, the distances are short enough and the utilization high enough that self-driving probably buys you very little over a taxi today. With the scenario you describe, the cost difference is probably greater. (On the other hand, if that's 50% of today's taxi costs, most people won't be using it to commute every day.)

Of course, there are still other cases where it's not feasible for most people to hire a driver today--say going away for the weekend because the driver costs start to dominate the vehicle rental costs. (And in that scenario, you just rent a car today if you don't own one.)

In any case, when such vehicles become available in however many years, they will probably reduce car ownership. But, as with Zipcar, I suspect the difference will be at the margins.