Most modern airliners only have a pilot and copilot up front. The flight crews are trained to work together as a team and to check each other for errors. What you suggest is to put all of the actual flight operations in the hands of a single person. That would require a complete change in how flight crews operate and are trained. I strongly suspect that would actually increase the chances of an accident, not reduce it.

I can support the idea of better isolating the cockpit from the cabin. We could use Kevlar blankets to make the cockpit bullet proof and put stronger doors between the pilots and any possible intruder. I don't see a problem with even putting private toilets, etc. in the area just outside the cockpit to further isolate the pilots. However, completely cutting them off from the rest of the plane could well increase danger in many circumstances. As a former avionics technician, I know any system can and will fail. Elaborate electronic safeguards on the autopilot is just asking to get clobbered.