It is not powerful then it can make the banks disappear? What are you trying to say? Let's say that bitcoin is not that powerful so it won't make the banks disappear. The government of a country is not going to agree if ever there's a threat of making the banks disappearing, because it is a very important body of a country.
That's correct Government will never support bitcoins as it can be threat to their national currency and no Government will be interested in seeing that any digital currency replacing its major currency and if that happens then Government will loose all their control from the money.
Don't say that government will "never" support bitcoin, there will be some exceptions that there would be time that they are going to support. But when it comes to threat of the whole nation, they are not going to show mercy on bitcoin. And I really can't see a chance that bitcoin will ever have the chance to replace banks in the circulation.
The Government might not accept Bitcoin as the only payment inside their Bank because the whole Bitcoin thing is kind of new to them. Since Bitcoin is doing many updating and bug-fixing from the users and the developers within Bitcointalk and other places, there should be a few Banks trying to have it added into their payment system to help it flow along with the regular Fiat within their Company. If there was Banks that only accepted Bitcoin they would really have to rely on the citizens of the world in order to survive.