Observer is an excellent investigative class because it combines it’s mass of data to find the evils indirectly. The sheriff, on the other hand, requires no deduction.
Sheriff gets a result saying whether or not their target is bad, observer gets a result that is incomplete; they have to use their wits to deduce anything.
I am not saying eliminate sheriff and paladin, I am just suggesting more investigative classes like observer, who don’t get a black and white answer