At least the four stars of the Southern Cross, which also appear on the shield, should be appropriate for our coat of arms, regardless of what the flag will do.
As to the cost, you can apply the reasoning that money can always be spent better elsewhere, to any concept which might seem a nice-to-have in a country which doesn't enjoy wealth. Any of us could apply that reasoning to our own lives, and frequently we do.
We know perfectly well the money spent on those new shoes could be better put towards the new paintwork the house needs.
A flag change might seem, in the context of growing as a nation, a small thing. But it's more than just the colour and the design. It reflects the same sort of confidence that allowed us to move from a colony of England, from a colony of New South Wales, from provincial government. We would not question, today, the decision to change from the English coat of arms in 1907, to our own design. Perhaps it is a luxury, but perhaps it is also a relief that we can proceed with some luxuries, and do something nation-affirming, rather than the mundane but required infrastructure. And the lament that our grandfathers fought under that flag isn't going to cut it. Our grandfathers fought for the ability to choose our future.
In fact, if we really want to get into logical fallacies, if this country really wants to save some cash, how about we all stop smoking?