Do you have any evidence of this or it's your opinion ?

Half the time I board a flight most of the first class is empty or non-existent, business is usually somewhat vacant and then there is literally hundreds of people paying $1500 for their ticket.

If you do the math, the vast majority of the flight is paid for by economy.

I invested in an airline for years and they talk about revenue per square meter in various cabins during earnings calls and press releases.

Economy (excluding Premium Economy) is the least profitable for the space.