It's the benefit of 20-20 hindsight. Sometimes when you dump a good coach for an unknown, the unknown wins, and you look like a genius. Other times, the unknown flops, but those cases are less memorable.
With a few exceptions like Popovich, I don't think there's any real difference between a good coach and a great one. The ones we call great are the ones that managed to get themselves into championship situations, or had the right voice for the right group of players at the right time. One of the great fallacies of sports, and of the NBA in particular, is that every outcome, once we know what it was, was the most correct or most likely probable outcome. If Team X wins the title, that's proof positive that they were the best team. It's a useful philosophy for bragging rights, because it's easy to define and there's no mechanism for challenging it. But to use it to create a mythical group of "great coaches" who somehow have a special magic that all other good coaches lack strikes me as naive.
To put it more simply, I don't think Monty Williams is anywhere near one of the biggest reasons that the Suns coughed up their 2-0 lead to the Bucks. If Saric hadn't gotten hurt, or Ayton hadn't lost his nerve, or Paul weren't a historic choker, or Antetokounmpo had needed just a little longer to recover from his injury, the Suns could have won. Would that have made Williams a "great" coach, instead of a merely good one?
