I think the idea of adding A320 capacity to JQ and opening up new bases and routes is to increase overall Group profitability....(hence mainline jobs more secure)....by offloading onto the lower cost JQ subsidiary routes which would otherwise sink into a sea of red courtesy of Qantas legacy cost structure and inflexibility.
So by doing things like opening up PER and DRW bases and routes, Jetstar makes money in its own right and decreases the losses that sister airline Qantas mainline would otherwise have made. Also quarantines the "low yield" virus away from the high yield focus of the legacy sister, thus further preserving mainline jobs while creating jobs in the LCC younger sister. Win-Win it would seem.
The whole strategy seems to work thus far, except to those who add new meaning to those old words...."There's none so blind as those who will not see"