I never think that Bitcoin will be able to replace the US Dollar because I just do not see something like that happening. The US knows what they are doing.
Bitcoin does not need to take over the US Dollar, it needs to walk on its own path.
I agree that it will never replace any currency not only US Dollar. The reason is that bitcoin is decentralized and the government will have a hard time regulating or controlling it. Aside from that banks and other financial institution will reject it because it nullifies their control and wants the centralized currency.