Why? The experience of not driving or looking for parking at all and doing absolutely anything else while you are transported is significantly better than driving a BMW. That's why the really rich people have chauffeurs. There will likely be taxi services that cater to the elite with better interiors, just like there are private plane services that cater to them. The vast majority of people driving mere 5-series BMWs will be happier with merely slightly upscale right-sized taxi services.

Because they can also change to a self driving 5 series BMW instead of Twizy. If they now pay the premium, why not with self driving cars?

They will then have to find a place to park the car, handle maintenance, etc. The cost premium of offering luxury vs. no luxury is in line with the cost premium of owning a 5 series vs. owning a Toyota. The premium of owning a car vs. not owning a car is not.