Of course it can't. Bitcoin is not accessible to all people specially the people who live on below poverty line. Value wise it is the most expensive currency but imagine going to the store buying things using bitcoin and waiting for a transaction to be confirmed. Hell no.
What are you talking. If the price reaches the value of 10k $,bitcoin will replace the dollar. FIAT of various type.First one is dollar. At first bitcoin will replace the dollar and follow by Euro and so on.But it takes some time to make replace the FIAT. You can trust bitcoin ,it will replace FIAT.
Replace in what means or way? Because I don't see Bitcoin becoming an official currency of any country and replacing their own Fiat currency on that matter. Mind you that price is not a factor in determining if it has the ability to replace a currency. One of the main reason is its legality their Fiat currency is their official currency replacing it by something that virtually exist and technically can be controlled by other people aside from their citizens is really a bad idea of destroying your economy.