Yes, J.K. Rowling should continue her famous Harry Potter series as there is a demand for it. There have been many fanfiction stories written about Harry Potter growing up & moving on with life beyond Hogwart's. The die hard fans of this series would love to see more from the author.
While I think it was great that there has been some time between the first 7 books, you can tell from Rowling's recent short story on Pottermore that fans are still eager to read about the wizarding world. I think she would be wise to continue writing the story, but to focus on younger characters, making it a new series, and not a continuation of the 7 published books.
Just as child actors struggle to retain success as adults, fictional characters often lose their luster with fans as they age. Early fans of Harry Potter liked the young teen characters. The level of connection audiences feel with young characters often wanes as both the audience and characters get older.
The original series was so amazing that anything that she could write currently wouldn't do it justice. The series also wrapped up very nicely and it would be difficult to start over where they had left off. I think it would hurt her reputation if she would decide to write more.