People have the extra money but they don't have to spend it on hamburger. They can spend it on durable goods as illustrated by the Discworld quote above. And just because demand for something goes up doesn't mean prices will. Often increased demand is a signal that a new market has opened up and this draws in competitors, causing prices to go down.

Look at the headlines about solar panels! Demand for solar panels is much higher today than it was 20 years ago and yet prices are dramatically lower.