When you strip out all of online and retail demand not caused by a catalog mailing, you stumble across an interesting rule-of-thumb (yes, your mileage will vary).
Across my projects, it is common for the top 20% of catalog circulation to account for 50% of demand. In the table here, 18% of the circ yields 51% of the demand.
This ratio used to be closer to 25% yielding 50% ... it is changing over time ... the disparity is getting bigger and bigger.
It means that as catalog marketing shrinks as a discipline, the gains generated by catalog marketing are even more disproportionate ... skewed to an even smaller number of high-quality customers.
Which means that all of us are responsible for finding alternative marketing tactics for everybody else.