Suppose that your company publishes 80% (by volume) of the Perl books sold.

Suppose that, as is the case, sales of a book drop by at least 50% every year after publication -- if not more. (Several factors contribute to this: limited shelf space at physical retailers, frontloaded markets that buy during the first three months, diminishing returns for advertising after the first three months, a technical book market that measures obsolescence by copyright date, et cetera.)

Suppose that your company stops publishing Perl books for a couple of years.

Suppose that your business analysts say "Hm, we're not selling very many Perl books anymore."

Would you draw the conclusion that you're seeing the results of your business decisions, or would you draw the conclusion that people just won't buy Perl books anymore?

Me? I find it easy to believe that a nine year old Camel 3 has stabilized in terms of weekly unit sales -- and that's not a large number.