yeah, there's always those times when you want to take over yourself, it'd certainly silence the "but i enjoy driving" argument as long as its an option.
wether or not it'll become auto only as a requirement is a matter of time, hopefully long enough so i don't have to deal with it, of course its an interesting question to ask if it has the ability to be driven manually it'll require a driving licence, which means one of the big advantages of driverless tech won't work- using them for people who couldn't otherwise drive eg: blind people, of course i suppose they could make exceptions.
That could be fairly easy to solve, if it's really an issue at all. For starters they just don't choose to manually take over the vehicle. Even today a blind person (or someone with a suspended licence) could technically drive, there's nothing stopping them from just taking keys to a car and driving it. The car currently doesn't know who's driving and neither do the authorities unless they are pulled over.
If we really need a way of stopping people setting the vehicle to manual that don't have a driving licence then it would be simple enough, even with todays tech. Just implement a system where the vehicle checks for a driving licence. A simple RFID chip in new driving licences and an RFID reader in the vehicle could solve that issue. If you really wanted to go high tech then a camera facing the driver with face recognition tech to make sure the driver is the same as the licence holder. It's all available in phones and bank cards today so not an unrealistic option.
Edit: or less complex, have the system set so some keys allow the manual driver option. It would basically just be another line of code for the ECU to process. Just use the same smart key many cars come with now, the same ones that auto adjust seats and set speed limiters. That way the kid can go to and from school on their own without being able to turn the car off auto, but dad can manually drive if he so wants.
Or have it set as a dealer configuration so if someone without a licence buys a car manual mode is disabled and you have to take it back to the dealer to be enabled again.
Lots of easy, simple, cheap/free options for this.