Besides Income Tax (in Australia) I’m not aware of any laws that force me to use AUD. If I decide to stop working one day (and get rid of all my investments) and live 100% off-grid – I would have to continue to pay my rates (land tax), but that’s about it. I could in theory just go back to a bartering system with my neighbors or local community and live that way. So i’d be living 99.8% without a fiat currency.