Yeh - but why Cargo?
"Driver is the depreciated hull cost and bottom line sums"
So why is that a driver for cargo but not for passenger?
Anyway let's consider that. If a hull is depreciated then it's cheaper for the company that currently owns it. Once it goes out on the open market it reconnects to its market value. If that value is low it is because the hull is unattractive to prospective buyers. Now why would the original owner or other passenger operators find the price unattractive whereas the cargo operators don't?
The answers so far are largely about old planes, not why old planes are ditched by passenger operators.
I have just one theory - older planes are less reliable, and the costs of delays for passenger operations are much higher than cargo.
I can't see how newness appeals to passengers. 747-100s were soon dumped for 200s and then 400s. Could passengers really tell the difference? The reasons we know were first fuel economy and then two-crew. Why don't these factors push cargo operators to new kit?