So what? The average depreciation you mention is real, but it also assumes average use. These are sitting ready, and presumably / should be being maintained (

So what? The average depreciation you mention is real, but it also assumes average use. These are sitting ready, and presumably / should be being maintained (e.g., periodically started, driven, fluids & battery checked, tires rotated or lifted to not flat-spot). And the intended use here is for their own driving. They really DGAF about resale value as cop cars are typically maintained in-house and driven until they are no longer useful for the PD, then sold for really cheap (two friends each bought one for ~$1500, and they still got a few years of use). So that typical depreciation you cite doesn't matter, and they save the entire pile of cost of switching over to new vehicles for at least one extra generation. As long as they store them in a place and way that keeps them ready and prevents rot, it's a good plan.