The big problem is it's VERY hard for people to stay continuously ready to take over at a moment's notice for long periods of time and then to maintain the disc

The big problem is it's VERY hard for people to stay continuously ready to take over at a moment's notice for long periods of time and then to maintain the discipline to do that over many trips. We just naturally get complacent and pay less attention than the systems currently demand because of their shortcomings.