That assumes trip demand is kept constant, which isn't a reasonable assumption over the long term.
Make it cheaper (in terms of time, money, effort, whatever) to drive and people will drive more.
If a car can drive itself, then trips that previously wouldn't have happened for lack of a driver will now happen. In the past, maybe some parents wanted to drive their kids to school but couldn't because they weren't available; well, with a self-driving car, the car can drive the kid to school and then return home - and repeat this again in the afternoon - all by itself. You now have 4 trips per day that weren't happening before.
Or say someone who lived downtown didn't own a car because they had no available parking. Well, now the person will be able to lease a parking spot somewhere else and drive wherever they please whenever they call the car.
And there will be a break-even point point for parking charges: if the hourly cost to park your car is more than your car's hourly operating cost, it would make more sense to just have it drive around while it waits for you instead of paying to park... and that hourly operating cost is dropping all the time as vehicle technology improves.
Even if there's a rule that says a licensed driver has to be in the car, there's still going to be more trip demand. It's basic economics that, all else being equal, as the cost of something goes down, consumption goes up.