"Nobody mentions that an economy has a hypothetical failure mode (like the game of Monopoly) where all of the money ends up in one person's hands -- and then trading stops."
I don't think the point is to offer this literally as a likely outcome.
If I interpret the parent correctly, the point is more to argue that there has to exist some level of inequality which even the most hardened libertarian would agree is sufficiently bad for the economy to justify an intervention. Hence reducing it to a question of degrees, a trade-off rather than a pure matter of principle.
Stunning level of ignorance.