I think bitcoin will not become a global currency and even more, any currency will not become global. Many peoples who don't like bitcoin or dollar or euro, etc
You are correct on the population but thats not how its defined I think. Both Dollar and Euro are reserve currencies which means they are carried by most central banks of the world as a major security to trade in global commerce, since a population is using the central banks reserve when handling their FIAT currency it means all those populations are in part using dollar and euro to trade.
Those two big FIAT notes probably cover more then two thirds of the worlds population which sounds incredible but consider for example the Bank of China holds some of the largest dollar reserves in the world and so is backing its own notes used by over a billion people with dollar also euro. These are very much global currency, its a long stretch to see central banks carry bitcoin.
I dont think its impossible but its some years off yet, even a big trading nation like United Kingdom with its sterling is only held as a small percentage. They have major centers requiring payment in sterling at least in part, so Bitcoin which can make no demands like this is not held as a reserve asset at all so far. Some high finance speculative merchant banks might have bitcoin or a bitcoin etf kind of deal , I know some banks are holding shares in gold mines as an asset to trade against. Famously JP morgan purchased a silver mine but also short sold the live traded price for silver on demand contracts. Large banks arent yet involved with bitcoin like that, its not global yet