Its actually the best choice for Australia: it buys them a bullet proof excuse not to send thousands of Australian lives for nothing. The Americans know only too well how much of a bitch it is to deploy these things overseas, and they are the ones with by far the most developed infrastructure to shuttle these things across the planet. If you can't send armor you can't send the rest of the army - they'll just die needlessly, so you have to scrap plans for boots on the ground altogether
You are missing the point... Australia is buying Abrams tanks and is getting US Nuclear powered submarines so Australia can become their new forward base for fighting China and Indonesia and possibly India if they don't make the right choices (as far as the US is concerned).
Australia buying Abrams tanks means they don't need to bring as many of their own and the local monkeys will be able to service and support their tanks for them.
It is the same with the nuke subs... if Australia only spent 60 billion on some French SSKs that would be ideal for protecting Australian waters from any threats, but would be useless for sailing up to Chinese waters and harassing the Chinese and North Koreans, which the Americans want them to help them do.
As an added bonus spending 400 billion over the next decade or two means Australia becomes an SSN sub hub for the Americans and the British (but not probably the French) so they can forward deploy their subs to the Pacific and the Indian ocean because Australia is right below Asia and next to the Pacific ocean and of course off to their left is Africa... a very useful location for the UK and the US to be able to operate from... with the bonus that the locals speak english and are essentially a white european based culture... which of course they would prefer to the damn natives in Japan and South Korea... who are not white european enough.