With all due respect to Mr. Lewis, this quote is crap. I have no reason to believe that the robber baron's cruelty may sleep, nor that his cupidity might be satiated, nor that conscience enters into it for that robber baron.

A private monopoly is accountable to only the owner. May as well be feudalism. In a liberal democracy, government monopolies have accountability to the people. When that is subverted or corrupted, it sucks, but at least there's a mechanism.

There's no such mechanism with a private monopoly.