It is not happening, bitcoin cannot replace FIAT banking system. You are talking about a system that has been in place for many years. Bitcoin is still relatively unknown and its potential is still developing. I don't see governments giving up control over money in favour of something decentralised.
I agree with you. Bitcoin is also too volatile at the moment.
From a buyer perspective.
Imagine that you only have Bitcoins and the value drops 20% at the moment you wanted to buy an ITEM. You will either pay more or you will wait another day.
From a seller perspective.
Imagine from a seller perspective, if there was a sudden huge spike in value of Bitcoin, as a seller you might not want to sell your item at that moment, because you will get lesser bitcoins and there will likely be a quick correction.
For Bitcoin or any replacement currency to be mass adopted, it can have gradual price movements, but current volatility do not inspire confidence to be used as a main source of exchange.