I don't think bitcoin will ever replace the US dollar. The government would not allow that to happen. I could see bitcoin being used as a payment method like paypal, or a method of sending money like western union, or even a way to store money instead of banks.
Exactly. Bitcoin will stay as an alternative payment method.
Though it is unpredictable but still i also feel that it may become more of what it is today but it would definitely not be able to replace any currency in the world as it is a free system which cannot be controlled and currencies running anywhere are in control of their governments, so it is really likely to happen.