If and when cryptocurrency finally becomes an official global currency, how do you think it will affect the global economy and the economy of each countries? Will it help improve the economy better than fiat or will it still need help from fiat?
It's very unlikely and pretty much impossible for bitcoin or any other decentralised cryptocurrencies to be declared officially as some sort of official global currency. Governments around the world will simply not be willing to give up their control over their respective economies.
If there was to be a global currency, then it would be centralised, and is essentially fiat currency, except run on a blockchain. That would pretty much be the same as right now, and not change anything in terms of how the basis of the economy operates.
If you're referring to the mass adoption of bitcoin around the world, then I believe that it's possible. Economies around the world wouldn't be affected much, as I think that it's very likely bitcoin will simply be an alternative currency form to fiat, that co-exists with fiat. It wouldn't need the fiat economy to exist.