If you're not using the feedback that you get after each sprint review to change the product you want to build, you're not doing agile. If you keep the backlog as a list of immutable things that must be done, you're doing it wrong!
The backlog is not the target. the PO should focus on selecting the items which bring more values, for the next sprint.
Inspired by Ron Jeffries' post "The Backlog" (http://ronjeffries.com/articles/015-10/the-backlog/article.html)