Is IT defeated in the end of the book?

IT is defeated in the sense that the Murrys are freed from the hold IT had over their minds and/or bodies. Further, at the end of the book, the Murrys are all returned to earth. At the same time, we have no assurance that IT's power over the planet Camazotz has been lessened or broken.
This could be understood as a flaw in the novel. The Murrys have saved each other and gone through, along with Calvin, a personal family drama that has ended happily for them all, but as far as we know, the people of Camazotz are still under the tyranny of IT. They are not allowed to think or make decisions for themselves. We have to imagine that the planet is still shrouded in darkness and that the people are still subjected to extreme conformity and punished severely for noncompliance. A more satisfying ending might have led to the Murrys freeing the people of the planet as they freed themselves.