Like advertising and maintenance,
you may be losing money, but if you don't do it, you can guarantee you will be losing a lot more money in the future
Seems like a relatively cheap way to increase capacity and passenger experience (particularly versus the competition) without spending $$$ on new aircraft.
The bulk of the older 738s were from the AA order that QF took after 9/11 so are approaching 12-13 years. Looks like sensible spend for an aircraft that will likely be with QF for another 5-10 years
Agree with goeing boing that it may also mean thinner seat cushions, or the newer slimline seats that provide more legroom