I'll go in reverse order;
- nostr isn't like an app that's controlled by a company. It's a protocol and anyone can build on it. So it's impossible to ban, it'd be at best a game of whack a mole
- agree, not yet the same volume. But things can grow very fast in this digital age. 
- banks can block access yet people are still finding ways. At the moment, not all banks have blocked access, credit unions allow access and some companies have ways that allow you to transfer without banks knowing what the transfer is for. There's also local or p2p ways to transact
- as for the ATO, if you have a bitcoin economy things may change but for now there are still plenty of options (eg/ you can get tax free loans out against your btc if you want to "cash" some out)