Which is more profitable, first class or economy class?

Generally speaking, economy class has a high profit. Because although the fare of first class is higher than that of economy class, the cost of decoration, facilities and personnel services of first class is much higher than that of economy class, and the distance between first classes is also larger than that of economy class, that is to say, the density of first class is far less than that of economy class for the same cabin space. It is also very important that the average attendance rate of first class is generally lower than that of economy class because of the cost. After all, first class is used to serve relatively high-level passengers. Think about it the other way around. If the profit of first class is higher than that of economy class, why not configure the whole plane as first class?