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.
This is not all about the World currency but it is, I am not sure what to say about it but my personal feelings are that Bitcoin should need to be Universal Currency as a store of value to achieve the height of its glory until that I can't say anything. This topic has been discussed several times about the valuation of bitcoin, future prediction, store of value, and much which are quite similar to the OP.
The point to notice here is without a globally leading country's support bitcoin can never be a universal currency such as China or USA and if one of them will support it then the other will resist and both countries can never agree on the same decision. From the past developments, we can see that whenever a particular region starts supporting bitcoin the other one resist as previously China was supporting and the US was resisting, and after that things changed China banned Bitcoin completely and the Western world accepted it. Now again Asian market is interested in the Bitcoin and other sides of crypto market and Westrent world is brutally resisting the adoption and law enforcement institute are suppressing it.