The answer is not straight forward. People have often suggested this, but its a very simplistic analysis of a much more complex revenue:costs equation.
Basically, each section of the plane generates different costs per passenger according to the amount of floorspace and weight they contribute to the flight.
Good airlines look to make a profit out of every single passenger relative to their share of revenue, floorspace occupied, share of weight and direct costs. Its true that business and first pax should generate more dollars each in gross profit, but they should also contribute a lot more to the cost of the flight than a Y pax. Simply put, QF probably seats 20 Y pax in the same space they would probably put 6 or 7 business passengers, so for the "real estate" share of the costs they need to be charging at least three times as much.
If you think this is complicated, remember that an airline accountant would tell you that my analysis is superficial! So the short answer to your question is that its an irrelevant observation for people to make and not a principle that should drive any airline executive.
A final point, which airlines are making money at the moment? If any, its the budget guys with no first or business class!