It is said that bitcoin will destroy the fiat system but i believe that if this happens the world will change completly because banks and a lot of things will go out of market.
I don't believe on what you have said. Governments cannot just let the fiat system get rid easily. If that happens, it will cause a big uproar. Implementing new systems will be difficult. So I think there will be two systems at the same time: the system for currency and the system for digital currency. Bitcoin depends on the demands. So it depends if almost everyone know bitcoin.