> successful because they had the will to keep going
In the first case it was not the will but rather the cash that the company saved during the good times under assumption that those would not last. With that it was not problematic to carry on during the bad times. And when competitors that hired too many engineers as consultants for the oil industry went burst it was trivial to pick up their clients. (In Norway it is rather hard to fire a person on a permanent position so a bankruptcy often is the simplest solution for a small company.)
> the person was not successful when they quit after the first setback
Only a very optimistic person would invest a few years of savings into something he had only vague idea about. And that fire on the shrimp farm was not a setback, it was a total disaster. As far as I know the farm was not insured against that type of damaged and the guy simply had no money to try one more time.
Note that I am not arguing that pessimism is required for a business success. I just want to say that even if optimism could be a necessary trait for some kinds of entrepreneurship, it may lead to failure just as well as to success.