We're not talking about a world that only has one law "you can do what you like with your own body".

Other laws still exist too.

Owning pillows is legal. Killing people is not legal. Just because owning a pillow is legal doesn't mean you can smother someone to death with it.

I would hope that someone diagnosed with a mental disorder such as Alzheimers would lose their drivers license (not sure if this is the case), but if they were subsequently cured and passed a new driving test, then would you have a problem with that?