What do you do when those 'low quality' users are sending rape threats to specific users? Or stalking them in this scenario? The police aren't going to do anything because it's barely an actionable threat and hard to trace.

The entire reason shadowbans exist and have existed for decades is because there's an inherently unsolvable problem with certain users that extremely fixate on certain users or communities. If you think they can be treated by being civil then I don't think you've been a moderator for any extensive period.