Earlier quoted context omitted.
The monopoly game is not zero-sum. Money increases when you pass start, and cards drawn are on average a source of money. In fact, the money owned by players inflates as the game goes on.
You could argue that's just inflation. Total real value remains the same with every lap, it's just distributed differently between players (particularly if you count the unclaimed tiles as an extra player)
Also players selectively mortgaging properties will destroy overall wealth in the game.