Why do they need to replace almost new A320's?
Remember that their main competitor in the Asia Pacific region has also placed a large order for A320 NEOs which would give them an operating cost advantage, so it was better to go ahead and maintain the status quo. As it is most LCCs get rid of their aircraft once they reach 7-10 years old, usually because they get pretty thrashed.
And as for "funded by mainline", by the same logic JQ helps to fund new aircraft in the QF fleet so everybody wins at the end of the day.