The government would still be able to tax people on income tax and GST if bitcoin does end up becoming adopted further, and that legislation recognises bitcoin as a form of currency. There however may be double taxing happening in regards to charging both GST and CGT when buying stuff with bitcoin.
Furthermore, I don't think that increased bitcoin adoption would lead to more people getting scammed at all.
The government would obviously still have its own set of regulatory framework in terms of bitcoin investment services, which would restrict ponzi schemes from theoretically operating. Obviously this isn't possible practically all the time, but I think that it's up to the individual to recognise whether an investment is of a pyramid of ponzi nature anyways. Regulators can only do so much in this sense.