I think one of the problems is that it will be more expensive for people to use bitcoin than fiat. We can also say that it will create a huge load on the bitcoin network to make all these transactions so it will probably be even more expensive and slower because of that. Also worth mentioning that bitcoin is still pretty volatile and I don't see it becoming more stable in the future. And the whole system will depend on the miners and if there gonna be any problem with them, then the system will suffer. And another thing is that when people will see how volatile bitcoin is, they will try to exchange it to more stable asset that will dump the price of BTC as well.
Why would Bitcoin need to become like USD or EURO while in reality it was created to be the exact opposite and to become an alternative mode of payment or store of money where no one can see or control the financial holdings or finances of anyone using it? Fiat currencies are centralized and under the full control of authorities of different countries and organizations. Bitcoin isn't one of them, it never was nor ever will be.
So it's probably better if we don't compare a decentralized cryptocurrency with traditional centralized currencies or even try to put it among them because it is different by all means and that's what it was created to be.