I think a good way to think about the last 5% automation problem is to look at other examples where automation has replaced human work. In every case that I can think of, the work of the human was never 100% replaced. Automated check outs at the supermarket still require a number of attendants. An automated assembly line still needs factory workers. Automated rail systems are still controlled by humans in a control room. You can also look at how the environment has been adapted to better suit the new technology. For example to allow big heavy horseless carriages roll along roads we replaced dirt with tarmac (asphalt).

Driverless cars will never (until we have general AI) be able to fully replace a human driver. However we don't need to get to 100% for the technology to become useful. It might be that driverless cars will be restricted to only driving on certain mapped roads. Or every car's movements are tracked in a central control room and if a problem is detected either by the car or the passenger it could be controlled remotely. Or let's say a GPS sensor breaks an actual human driver might be issued to come and rescue you. Lastly we might even accept a certain failure rate. Society is willing to tolerate failure of technology to a certain degree in cars and planes today. All we need is is for the technology to make us all better off on average for it to be successful.