New userstories that appear in the backlog mess up the backlog organization. If they would appear rather in an unorganized pre-backlog (or whatever) then it would be easy to see which items have not yet been assigned a priority. Also the PO could move items that are in wrong spots in the backlog to the pre-backlog as she is prioritizing it and then re-prioritize the wrong ones.

Hard things like backlog prioritizing are done by "Divide and conquer". Please let me divide by implementing a pre-backlog.