Australia makes and exports different things to the USA. For example, there are some parts of their bomber planes that only an Australian factory makes. If we stopped trading it, they couldn't buy any more of those bombers.

There's also the impact of solidarity, where if we stop trade, then other countries are more likely to do the same. This effect has happened with previous blockades and with the general sentiment towards the conflict in Western governments, one country takes the first step and others soon follow.

It also means that, as smaller partners stop trade, the US soon becomes a single point of failure, where a port strike or community blockade (as some in the USA have done before) has more direct impact.