Demand is not a single number; it is a function of price. You can fill any route with a 747 at the right price.

Given today's lower yields, you might find that 747's are more useful than they were in the past. UA just retired the 727, and they're flying more 747's domestically. A 747-400 has the lowest seat-mile costs of any plane. The key is to put them on high traffic and/or low yield routes. SFO-LAX is high traffic (think rush hour). Anywhere to LAS or MCO is low yield.