Necessary but not sufficient right? I alluded to the same point in my comment above, successful entrepreneurs get shit done and don't make excuses (ie. if something is a true problem then the find another angle).

However there's a significant selection bias at work in believe that tenacity always wins. In reality there are no doubt many people who simply do not have what it takes. Granted most of them do quit rather than spending an entire life trying, but I'm sure they're out there, you just haven't heard of them.