Detective fiction turning into police fiction sounds interesting, because it mirrors amateur turning into professional, in sport, which happened remarkably late, in this country, and has (as you would expect) all sorts of race, sex and gender expectations tied into it.

Look at it; you have the "gentlemen amateur" (and occasional lady; Miss Marple etc) tradition of investigator replaced by people who do it because it's a job; where you contrast someone who doesn't have to worry about who minds the kids and the cats when they hare up to Aberdeen to follow a clue with someone whose partner hates the job and who is always having to negotiate work/life balance issue, one becomes "realism" and the other becomes "fantasy" (for strictly limited values of each).