so are you saying one has to be president before one is president? or be 'promoted' from VP to president? if this were the case, you'd have to be president of a multinational corporation, a 5 star general who has been in at least two wars (one global scale, one small scale), have been through a depression, have been through an energy crisis, have been through blah-blah-blah all before you are finally qualified to be president.
no president has seen it all, be it before their presidency, during it, or after it. every president, republican or democrat, has received on the job training. some do a good job - for example, JFK had no training for the missile crisis, i'd say he did a good job dealing with that. some do a bad job - for example, jimmy carter boffed the economic crisis he had to deal with.