The core of the issue is that a manager (President is just ultimate form of it) working in a peaceful and prosperous time period is not needed. He can be replaced with an automated set of scripts and mostly nothing would be affected by that replacement. In fact, we can't even correlate person's skills to a company/country performance if it was successful. Maybe that manager was incompetently hindering it, and it would have been even more successful without him.
Manager's skills are only shown during a conflict or a crisis. But no human has invented a way to test candidate on the crisis response. Partial solution is to hire/elect a person who manager crisis before, but because crises are always unique, their skills still don't translate to a new one.
So the answer is - it is impossible to do. Thus we shouldn't do that for the most critical positions.
Manager's skills are only shown during a conflict or a crisis. But no human has invented a way to test candidate on the crisis response. Partial solution is to hire/elect a person who manager crisis before, but because crises are always unique, their skills still don't translate to a new one.
So the answer is - it is impossible to do. Thus we shouldn't do that for the most critical positions.