Bitcoin won't replace fiat just like credit cards haven't replaced cash and PayPal hasn't replaced credit cards. Bitcoin will eventually supplement existing currencies.
You are comparing currencies to technologies. Sure, Maestro Cards didn't replace paper Euro notes, but paper Euro notes replaced Italian paper Lira notes, for example. Bitcoin may not replace credit cards, either, but that doesn't mean we can't end up with a Bitcoin denominated credit card that replaced dollar and euro credit cards.
I suspect that any particular crypto-currency is more likely than not to succumb the the same sorts of pressures and corruption that most forms of wealth fall victim to.
Can you elaborate please? Government and private issued currencies fall due to the issuer falling. Bitcoin has no issuer. Gold and other "hard" currencies fall because they are difficult to store and transact with, and the places trusted to store them end up either succumbing to fractional reserve or to the currency being diluted (like with tungsten found in gold bars). Bitcoin is easy to store and transact on people's personal walkers. So what kinds of pressures and corruption do you have in mind?
Regarding the OP, and the couple of replies after mine, ignoring my point, it's not "Can it replace...?" it's "Can it outlast...?" Do you guys believe Bitcoin can outlast government fiat currencies? If yes, why can't it supplant and replace them?