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?
Freight hardly has lower utilization.
My operation sees the airplanes in continuous use. One crew gets out, another crew gets in, and the only time the aircraft sits on the ground is long enough to be unloaded, loaded, and fueled. Ride-on mechanics live aboard. The airplanes make a global circuit every two and a half to three days, and keep doing that until either required to stop for a field maintenance condition, or the aircraft returns to the mx base for a scheduled inspection or service. Otherwise they're continuously attended in the field and keep flying.
Hardly lower utilization.
Our aircraft are a little older, a little higher time, and not as efficient as newer aircraft of the same type which have entered passenger service. We get the airplanes when the passenger operations are done with them. They're less expensive than a new aircraft. With lower acquisition cost and lower overheads due to low or no aircraft payment, trip profit isn't diminished by large aircraft lease or loan payments. Many systems are removed or deactivated that might have been for passenger convenience. The aircraft is modified to carry freight. Freight doesn't care if the airplane looks older or tired. We don't have to polish paint and replace with fresh; it's a working airplane that largely flies in the dark, largely behind the scenes, and has only one mission...get the freight there.
Passengers do indeed prefer newer aircraft, and prefer clean, shiny airplanes without wrinkles, without a great deal of wear. They feel safer, they're more inclined to buy a ticket on the airline on which they feel safer, and overall maintenance costs may be lower in the long term.
For a freight operator, the acquisition of the airplane may be it's last life; it may serve here until it's out of cycles. I'm not certain I understand the dilema here. The operator elects to use the aircraft the operator elects to use. Why concern yourself what aircraft that is? It's a business decision.