Definitely, I am agree with you, when i know about bitcoin deeply, then i also have thought that bitcoin couldn't change in currency. Bitcoin would be faced too much difficulties if bitcoin become into physical appearance. First of all, i am not thinking about it now and also in future. People like to use bitcoin as it is.
My point of view for this, bitcoin cannot replace the world currency for now because price is not stable. You were right on this, its not stable and it has normal fluctuation of its value ever since when its started to hit in the market. I dont think it will replace the currency in the world as it is not yet accepted globally.
Bitcoin wasn't even created for replacing fiat currency. Otherwise its supply wouldn't be limited or would have a lot more than 21 million. Bitcoin was created to coexist with fiat and gave this world a new form of monetary system different from centralised form of currency or financial system.