No current routes would support a jet against the economies of the current fleet.....
Therein is the rub. If the Link ends up with A319s they won't be for current routes. They'll be for routes where QF wants a premium presence at a lower cost.
The next thing that may happen is if the link gets A319s then A320s and A321s aren't will be the logical next step in the medium to long term. There's the replacement of the mainline 737 fleet in the long term at costs more than a 1/3 cheaper than current rates.