Bitcoin is already a global currency, but replacing USD as a global reserve currency is a far-fetched theory and for that to happen the extent of Bitcoin adoption should be exponential, major countries should adopt and legalize Bitcoin as currency, one of the primary currencies for trade throughout the world should be Bitcoin. If Bitcoin becomes a global reserve currency then obviously the price would skyrocket. In terms of functions of money:
1. Medium of exchange.
Scalability, volatility, transactional cost, speed, fee matters. Assuming Bitcoin scales to accommodate large scale global transactions and there is more price stability, Bitcoin would be a reliable medium of exchange.
2. Unit of account.
To serve the function of a unit of account, Bitcoin should be able to measure the relative value of goods and services and other transactions in the economy. Divisibility and volatility matters. Since Bitcoin is divisible it enhances its unit of account function, but volatility can be a major con especially if there is short-term price fluctuations. A fluctuating global currency makes it difficult to spot the true relative prices of goods and services leading to adverse effects.
3. Store of value.
Since Bitcoin is deflationary, it is beneficial to Bitcoin holders, good store of value, but at the same time the expectation of higher future value of Bitcoins may lead to its hoarding in anticipation of higher price which in turn may reduce its use in exchanges of goods and services. Although increasing and retaining purchasing power, its use as an effective medium of exchange might be limited.
From an economical perspective, according to Blocklink analysis, the richest 30% of all Bitcoin owners own 98.3% of all Bitcoins. So the poorest 70% of Bitcoin owners own less than 1.7 % of all Bitcoin. It takes merely 0.153 Bitcoins to be placed in the top 30% richest Bitcoin owners. Assuming Bitcoin becomes a global reserve currency, the wealth distribution still somewhat remains the same as the existing system.
Bitcoin doesn't have to replace the banks or Fiat or become the global reserve currency to be disruptive. It just needs to accomodate and offer alternative solutions be it storing wealth or P2P transactions to people who don't want to use banks/Fiat.
http://www.blocklink.info/distributions.html