Today many countries accept bitcoin as a virtual currency, is it because bitcoin is so high that they think bitcoin can change the country's economy, or because the country has many bitcoin users already?
There is a chance that bitcoin could improve the economy of a certain country. For one, all the bitcoin earners living in that particular country would probably use their earnings from bitcoin to buy things within that country. Those things would surely have taxes. On that process, bitcoin is improving the economy of a certain country with the help of it's users.
Yes, you're right. The only way to make bitcoins contribute in a country is to use it as a usual currency like fiat. If all of the merchant accept bitcoins as a payment. They use bitcoin to buy product or services in which could be obviously contribute with the GDP plus the taxes from those products (but still, it depends, more factors to consider)
Although we all understand the problem of Bitcoin to be legally is the tax, but do all the Bitcoin's user accept that? I don't think those who knew the Bitcoin's function will accept to spend money for the tax if they want to evade the tax for their business.