I think btc to increase a country's GDP or change a country's economy may be difficult for now because the anonymous nature of BTC will make it difficult to calculate into a country's GDP, especially to measure the economic level of a country that requires a detailed calculation, the nature of BTC this anonymity also makes many governments make rules against BTC to avoid money laundering
Bitcoin is highly volatile and can not control by any one. Fiat and bitcoin and never live with each other. If you once use bitcoin you'll never use your fiat because its tax free and no one want to give any taxes.
Its impossible. First of all not everyone aware about bitcoin or cryptocurrencies and those people who are aware about it they're not ready to invest. Government will never let people to use bitcoin due to there disadvantages and criminals activities.
The governments are in no position to allow or not allow the use of bitcoin as a currency, people are going to do what it is necessary for them to survive and we have seen this in countries where cryptocurrencies have been banned and people have decided they will use cryptocurrencies despite the ban because they will do what benefits them the most, governments despite their best efforts to control their population will be unable to control cryptocurrencies.
If they managed their fiat currency and their economy in a better way instead of trying to take advantage of their citizens all the time we will not have any need for cryptocurrencies in the first place but since that is not what they do then cryptocurrencies could take over in the future.