It is said that bitcoin will destroy the fiat system but i believe that if this happens the world will change completly because banks and a lot of things will go out of market.
BItcoin will be used by most people by 2020,but not by every one.There are still majority of world population in under developed countries who don't know about digital transactions.It would b hard for them to change to bitcoin transactions and it would take more time for all people to change to bitcoin transactions.