Bitcoin will never replace USD or any other currencies(unless possibly for a small poorly governed country), it may take a little of the value out of it, but not much.
It's more like adding another radio-station, it's not that important for most people, but very important to those who finds a new station they enjoy.
It is unclear whether or not you also believe that USD and other fiat currencies will last far into the future? It's important to note that fiat currencies are theoretically unsustainable. If one person or institution has the power to print unlimited currency, eventually we reach a point where that institution realizes its almighty power and destroys itself by debasing the currency to a point where people just stop using it. Did you read that book "If you give a mouse a cookie" when you were a kid?
It starts like this:
If you give a mouse a cookie (Give a central bank power to print money)
He's going to want a glass of milk (The central bank will gradually become more and more corrupt and print money as a band-aid to poor fiscal policy, debasing the currency until it inflates to nothingness.)
We're in the middle of that right now. Currently, global monetary policy coordination involves the speed at which various currencies are debased. Right now JPY appears to be taking the lead (in terms of planned printing), much to the chagrin of other Central Banksters. If any one country prints significantly more than the others, it will achieve significant trade advantage in terms of its exports vs. those of other countries. So it's like a "race to the bottom" where each of the central banksters doesn't want to let any one of the others get too much of a lead, but really hopes to get a lead him/herself.
Now, Bitcoin itself may not specifically replace USD
when USD(/JPY/GBP/EUR/etc.) fails (because it's not a question of IF.) It may be a combination of precious metals, notes tied to them, Real Bill-type systems, various cryptocurrencies, barter, scrip, local economy, etc. We live in an interesting world and people are very adaptable. There is not one answer to such a complex problem. So I agree that Bitcoin will not replace it solely; I just don't live in the fantasy world where Fiat currencies go on living happily ever after and things like PMs and cryptocoins and Real Bill systems (and localized freeconomies) take a tiny piece of the pie.
Fiat currencies are a failed experiment in economic history
PMs we can say are a success, because even after modern culture removing most of their utility as currency partly due to the "nix"ing of the gold standard, they still have significant value which has outperformed any and all fiat currencies in existence. Even taking into account storage costs, theft risk, selling 20% below spot, etc - they STILL outperform the dollar in the long term by a long shot.
Digital P2P Cryptocurrencies are still being tested; you are one of the test subjects
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