They will only have more money to spend with. Let's just admit it that only few of the government in the world are transparent. Some are very opaque to the point that they have a lot of money but the people are still blinded with their statue. If they put tax just to legalize the crypto, well, I can say that the crypto world would weaken because of them.
I think that it will be in favour of bitcoin and other crypto currencies, I think bitcoin need to be legalize by government and it can only be possible if they will put tax on bitcoin, may be in Japan they are collecting tax on bitcoin income because they have already legalize bitcoin.