NPV's are about projects Frozo, and when you do your NPV you also have to take into account costs.
What has been discussed and suspected about Qantas is that "The Qantas Group" is a fiction, Jetstar is profitable because the accounts are structured to make it appear profitable, and showing Jetstar making money is a convenient way to hide the enormous profits Qantas mainline makes from gouging the flying public.
There is a book you should read (probably long out of print) called "Up the Organisation" about the rescue of Avis many years ago.
One of the things management contemplated at the time was setting up a separate "discounted" car hire operation to compete against the cheaper discount operators without compromising high revenue corporate account sales for the main Avis brand.
The project looked good on paper until the marketing manager said "In Poland, we call that pissing in the soup".
The Jetstar operation is "pissing in the soup" in marketing terms. And if Qantas think that starting Jetstar and outsourcing engineering pass for a good corporate strategy then good luck.
P.S. The more usual name for "pissing in the soup" is cannibalisation.