Well in fairness, Mary Poppins was being made into a movie not a theme park section. But I see the comparison. But if JK didn't maintain the control she did over the movies, we may have seen an animated version or worse. So her judgment about her property has been pretty right on. The movies could not have been better.
It was on the table for Disney, from what I understand, it was she who refused. But, like you said, it worked out.
I will say that since HP is a children's book, I cannot imagine Disney not wanting in incorporate some type of character meet and greet. If not Mickey then perhaps house elves and the main characters. Either way not what she wanted.