I do not think it could be used as a world currency, because its value fluctuates frequently. I hope it is still a high value currency and is a long-term investment and a great profit for everyone.
Why do you think bitcoin is fluctuating that much? Beacuse its very small market. Imagine this numbers:
30% of bitcoins are lost (wrong adress during transfers, lost keys etc)
part of them are on cold wallets - long term investors or peapole that are using it (yea it can be usefull). In japan you can go shoping with only bitcoin wallet.
So lets say 50% of them are making price on market - 8 000 000.
Now let see how much money you need to came to this market and put bitcoin price to 20 000$ form 8 000$ in one day
20 000 * 8 000 000 = 160 bil - there are people with this amount money - single person can set price to 20 000 $ tomorrow. (160 bil is maxiumim (he will buy all bitcoins on market). If price will skyrocked lots of outsider will jump in and i think than less then 10 bil will be spend to this jump)
Now imagine how this numbers changes when mass adoption comes in. Bitcoin slowly go to 1 mil - 5 mil $ in next 10 years. Istitutional investors jump into market. It wont move that easyli anymore . It will need amount of money that noone is possible to spend to move price 1-5% (like other currencies on forex)
the only explenation of bitcoin fluctuating that much is size of market.