I remember in my ATPL's something along the line of passenger aircraft can only be used for so many cycles
I suggest not .. with limited exceptions over the years, the standards for pax/freight are the same.
So why is that a driver for cargo but not for passenger?
Competitive market pressures ..
(a) priority/high yield freight goes in the pax hold
(b) oversize (especially high priority/high value) freight may warrant a one-off charter
(c) the remainer is in competition with marine (international) and bitumen Boeing (domestic) transport
The accounting penalty of needlessly high capital hulls is not attractive .. however, as the price of fuel increases, the breakeven point will move toward newer, more fuel-efficient aircraft.
I guess that it's the same in other countries .. in Australia for as long as I can remember, priority airfreight will go via bitumen Boeing if the freight company can make the delivery .. there was a celebrated road accident years ago where a particular major freight company suffered the embarrassment of having a truck load of overnight air freight strewn about a major interstate highway.