If we had a government who supports bitcoin surely that support will come with some rules for bitcoin.That is mean bitcoin will not be decentralize anyone,We have to accept what they put the law on bitcoin.May be they will create a central bank for bitcoin.More more bitcoin they get they will able to control the market.Do you need that? I don't think so.
It obviously contradicts the whole idea of decentralization.
The usual reason why people kept on using cryptocurrencies is that they are free. They are not inside the shackles of the government, spying over the transactions they made. The transaction might be seen in the blockchain but you can only see their addresses but not who they really are. That is what we needed especially on the internet.