you don't need a blockchain to decentralise anything. blockchain only makes sense in a case where there is no central authority.

The ASX is already centralised.

A Stock Exchange is the antithesis of a distributed system. The reason they exist is so it's a central place where everyone goes to buy and sell and the prices are public and fair. And a clearing house is by its nature a centralised system (A owes B $100, B owes C $100, so A gives C $100)

That's why the majority of crypto is bought and sold at crypto exchanges rather than on the blockchain.

Mirror the DB across two servers, boom decentralized achievement unlocked

Speculating against the national currency is legal business. And blockchain affords the existence of cryptocurrency, which is one way of doing it.