The issue is, the DVLA don't listen.
My next door neighbour had cancer, his treatments left his head tucked permanently down to his chin. He was so weak he could barely move his legs, he couldn't steer properly and wouldn't give up his driving licence. We reported him to the DVLA and they couldn't care less. They didn't even assess him.
He continued driving when he was on incredibly strong painkillers, and only stopped when he was on end of life care. How he didn't kill someone I don’t know.
You can't have a hard cut off. But there needs to be more robust procedures in place to stop this kind of thing happening. Driving is a privilege, not a right, and we need to be tougher on poor driving from everyone.