And if you do assume "fiat money is doomed, doomed!" then why wouldn't something like bitcoin become the world's reserve currency?
Because governments would prefer other currencies. They would even prefer a gold or silver restandardization to Bitcoin, since bitcoins are harder to track than movements of physical metals. As far as spontaneous restandardization, (i.e. without government approval), metals have the advantage of an industrial price.
Tangential, but a subject of some local interest:
Why Bitcoin will fail by Avery Pennarun. "The sky isn't red." Thesis:
I'm not sure I buy these and am not competent to evaluate his claims on 3., but would like others' critique.
L019: Bitcoin P2P Currency: The Most Dangerous Project We've Ever Seen by Jason Calacanis. A rather more enthusiastic viewpoint of the project:
The actual text contains many more caveats than the eye-catching selection of points above.