The official explanation for the banning of cash is that it will stop the so-called black cash in hand economy, although I doubt it, since with e-payments and Bitcoin, I imagine that there would be high-tech ways around it. Maybe pay in tins of baked beans. i.e barter? Anyway, here is something published at their ABC, which is worth thinking about, that it is all about controlling your economic behaviour during recessions. How about just plain old controlling your economic behaviour, full stop?
https://www.abc.net.au/news/2019-08-26/cash-ban-so-you-pay-the-bank-to-hold-your-money-what-imf-wants/11443646
“As the Federal Government moves to ban cash transactions above $10,000, there's a theory gaining traction that the real motive for the cash ban isn't the so-called "black economy", but rather, to give authorities greater control over your behaviour during recessions. This theory, put forward by economists such as John Adams — and picked up by some federal politicians — has not been plucked out of thin air. It is based on repeated public papers and statements by the international body in charge of financial stability — the Washington-based International Monetary Fund (IMF). A recent IMF blog entitled "Cashing In: How to Make Negative Interest Rates Work", explains its motive in wanting negative interest rates — a situation where instead of receiving money on deposits, depositors must pay regularly to keep their money with the bank. As the blog notes, during the global financial crisis central banks reduced interest rates.