If you are saying cargo operators find older planes equally cost effective, or cheaper, because they have lower utilisation, then that begs the question why they fly less. If passenger operators like to keep their aircraft in the air, are cargo operators lazy or just thick. Surely they should buy new aircraft, get their utilisation up and thereby follow the pattern the passenger airlines have found to be optimal?
There have been various statements about why you might choose an older airliner but none explain why this seems mainly to apply to cargo and not passenger.