Although Satoshi Nakamoto designed Bitcoin as a currency, it is not practical to use Bitcoin for that purpose if the user base grows to billions of people. Bitcoin is having a fixed block size, and despite the new innovations such as Lightning Network, we are still facing delays in getting confirmations as a result of that. There is no long term solution for the scalability issue. We can't increase the block size indefinitely. If the number of active users grow from a few million (which is the case currently) to 5-6 billion (the number of fiat currency users), then Bitcoin will be needing at least 1,000x block size. And I don't think that the Blockchain can handle that.

So for now, I think Bitcoin will be best used as an investment asset. The controlled supply principle means that Bitcoin is ideal as a store of value, and for investment purposes. For usage as a currency, I have a feeling that currencies such as Ethereum will be more suitable, since the time needed for confirmation is low. But lately, I heard that Ethereum is also facing scalability issues.