She knew!

J.K. Rowling wrote in the first book about Harry Potter:

"A letter?" repeated Professor McGonagall faintly, sitting back down on the wall. "Really, Dumbledore, you think you can explain all this in a letter? These people will never understand him! He'll be famous -- a legend -- I wouldn't be surprised if today was known as Harry Potter day in the future - there will be books written about Harry - every child in our world will know his name!"

I know she began to write this books for her children without any expectations. But her prediction about success of the boy with a sharped cut like a bolt of lightning has turned out to be true.