That is not correct.
You can't pick just one single item, that is absurd.
Look at the evolution of the seat, the IFE, the lounges, etc.
I remember very well the old first class, and the pain in my back after a long-haul from the terrible seats.
Also the conclusion no first class compartment equals less yield is not consequential.
Where is your data to support your claims?
PS I am as much Paul as Curtain Rod is Rod.