It doesn't appear to mention it in that article, but one aspect that gets brought up a lot with regard to remaking the police departments is replacing the union. In many cases, it's the local police union that's actively working to prevent investigation or prosecution of police misconduct and crimes.

Firing and re-hiring everyone, legally and contractually, lets you get rid of the existing union and work on building a new one that's actually focused on what a police officers' union should be focusing on: protecting good police officers from abuse and corruption from above, negotiating salaries and job conditions, etc., much like any other workers' union.