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.