But there seem to be holes. Unruly armies of tens of thousands have existed since antiquity as insurrections, unrest due to demands for change, and borderline rebellions. The ruling classes of the Roman empire had to deal with this sort of thing on a regular basis. The size of the mob is not a new thing.
I think the change between old and new enforcement of common law has a lot more to do with technology, specifically the development of firearms and their ability to enable a small armed group to defeat a large unarmed group. That changed the whole nature of insurrection and civil disorder. You'll see that the advent of the police came right around the time at which firearms moved beyond the crude, slow weapons of the Napoleonic era. I don't think that is a coincidence.