Yes, she is a great writer. But would you say she's the greatest? Great and greatest are two entirely different things.
Babe Ruth is considered the great ball player ever because of how many years he played amazing. I know you probably don't know much about baseball but Ken Griffey Jr was THE player during the 90s. Then after injuries and average numbers, people question whether he is the greatest, because he was amazing then slowed to a crawl. You could say the same thing about many great directors today. They've put up amazing numbers, had career seasons (movies) but they could just as easily make Reindeer Games as their next movie. Or they could make the next Citizen Kane or Mr. Smith goes to Washington.
I agree one work is all you need to be remembered as great. But to be the greatest you have to do it for sometime.