Not sure what you mean here. If people started buying Bitcoins more then they'd be selling USD (or other currencies) for them. The less people wanted USD the more you'd have to give to get Bitcoin. Getting rid of your USD is something that makes USD less valuable. The same principle holds for other national currencies. But Bitcoin has a long way to go before it would make an impact - heck, they're already devaluing the USD at a far more alarming rate than any Bitcoin effect.

The important thing is that people have an alternative so that as USD become more worthless, and more irrelevant, all of the smart folk are already happily using their Bitcoins instead. And the same goes for any other national, centrally planned currency.

Imagine a future where the greedy rich banksters are left holding bags of currency that most people no longer want to bother with because most regular folk are already fully functional (and internationally) with each other using Bitcoin. Seems far fetched but stranger things have happened.

(crank up film reel of pre-war Germans pushing wheelbarrows of cash to the grocer)