Macaulay is saying, quite simply, that this kind of selfless action can only happen in a society that's well. Where people give a shit about each other. Where the poor are maintained, and the rich (and those in power) know their place is to serve. Picking up a little later: ---

Now while the Three were tightening Their harness on their backs, The Consul was the foremost man To take in hand an axe: And Fathers mixed with Commons Seized hatchet, bar, and crow, And smote upon the planks above, And loosed the props below.