> The reason that the rich were so rich, Vimes reasoned, was because they managed to spend less money.

The premise is just false. The parable might be true when comparing, say, lower class vs lower-middle-class, or lower-middle-class to middle class. But the difference between upper class and middle class is not "spending less money." It's a vastly different net worth that comes from inheritance, building / running businesses, investments, etc.

The boots theory focuses on the costs, but the real difference comes from the income & net worth