Bitcoin may be transferrable, but its use as a liquid currency is limited. Therefore, it will not replace other currencies.
It can be useful for someone with a bank account in a foreign currency that wishes to avoid costly bank transfer fees. However, its utility as an everyday currency is limited. For example, a tourist visiting Italy from the UK cannot buy Bitcoins ahead of their holiday and spend them across Italy. They would still need to exchange them for euros, either at an ATM or by depositing them into an Italian bank account. This severely limits the cryptocurrency's utility.