Like you, I find it implausible that the threat of being fired is the only good way to motivate workers. Indeed, for many jobs I don't even think it's a good way.

I think the best-run companies are well-run by virtue of individuals in positions of influence who are good at running companies. Hiring good people is a symptom of that, as is having a great corporate culture. A poorly run company won't suddenly get better if the CEO decides "Oh, I know! Let's hire the best people and have a great corporate culture!" unless said CEO develops skill at running companies.

Regardless, thanks for answering my question.