This assumes that generalists are equally effective at all tasks as specialists. Well, as a generality; in the specific case of all-purpose use of police vs appropriate use of other community services, it actually involves the assumption that specialists in the application of violence to achieve compliance are as effective in specialists in tasks unrelated to application of violence in those non-violent tasks.
In technology, if senior IT management decided they could reduce staff by having network installation specialists, with little to no additional training, cover application development, QA, requirements analysis, SRE, DBA, desktop support, and project management tasks instead of having specialists in each of those domains, they'd rightly be viewed as insane. But that's, broadly, what local governments have done with city services, with cops in the role of the network installers.