Bitcoin is a long way from becoming a currency you can live your life in. You also only have 7 transactions a second. But that doesn't mean Bitcoin isn't immensely valuable as a medium-term and long-term store of value
Honestly it is true that bitcoin cannot replace currency in the world because price is not stable just like the exchanges money to other country. I believe that eventhough bitcoin price is not stable it would not be a currency in the world.
The best thing is that bitcoin does not have to replace the currencies of the world, it just needs to be an alternative for those that do not want to deal with the banks and all of their rules and regulations, they impose those rules only to benefit themselves, bitcoin on the other hand benefits the people, and that is why even if adoption has been slow it is going to keep growing as more people see the light and adopt bitcoin in order to benefit themselves and not the banks.