> becoming a functional currency
None. At least until a sovereign nation decides to ditch fiat for crypto. The first to do that, I bet, will still treat it like fiat and use all the classical monetary policies. So, it'll be crypto in name only.
Eventually, IMHO, Bitcoin will eat all the money in the world, stabilize at something like 10-15 million per. Then it'll just kinda take over as being more trustworthy than any fiat. Of course countries will try to regulate the heck out of it. But, since it has no president of Bitcoin, it'll survive.
Then, we'll have to deal with the knock on effects of a deflationary currency and all that entails.