Ok i shall try to explain this as good as i can.
Lets take KLM an airlne that opperates bothe 744 pax and 74M (combi) as an example.
From what i know, both aircrafts are used on flights to JFK. But if the had to only opperate either the 744 pax or 74M into this destination (or only focusing on one type in their fleet) which do you think gives the highest income?
To make this a little easy the 744 pax is full with pax, and the 74M is full withoth pax and Cargo (so not many empty pallets or seats)
Actualy I belive the 74M will "winn" since from aht i have understood a high number of seats on pax flights (all airlines and a/c) are giving very little income since they have discount fares non revs. etc, and only a few full price pax.