If US Dollar crashes then Bitcoin price will go very high and people already holding it will make money.
I have to disagree sorry. Every decade or so a national currency crashes and its people are made poorer generally. Money does serve a purpose, a utility and some work is saved by allowing that convenience and ability to save.
IF that system is destroyed, generally losses occur. I agree prices will go up as the dollar worth goes down, its true but overall the effect makes people poorer as confidence falls and uncertainty means less trading.
Studies into the decline of the Mexican peso and its standard failure for note holders were made over years by Marc Faber who has traded internationally since the 1970's. He has a long slideshow and talk on this, his conclusion was that share holders in mexico were roughly even after 10 years. In a decade, they saw no growth occur and no losses to their original money.
We all know a country that has sucess can become richer and grow alot in ten years, so no growth is very poor. Inflation, currency failure is a negative.
The only point to add for Bitcoin is that its new technology. Some very clever innovative people want to make it better, if they improve utility its still possible you do gain in some way. This gain would come from world commerce increasing and the decline of a Washington centred monetary biased system, maybe!