I suspect it's partly because of the difficulty involved in measuring the quality of a given API's design. How do you separate the success resulting from the us

I suspect it's partly because of the difficulty involved in measuring the quality of a given API's design. How do you separate the success resulting from the usefulness or popularity of the technology from the design of the API? That was the first thing I though on starting to read this. How do they know they've actually got it right? Feedback from users?