This exactly!
The old school tech in me loves the idea of cryptocurrency, but really all it is right now is a "don't get caught holding it last" gambling .. I mean speculation .. game, with the more popular ones like BTC and ETH constantly being manipulated by pump-and-dump large investors to drive up the price.
I regularly see people claiming random cryptocurrencies of the moment will be the "future of global currency", but not without regulations on a global scale. A lot more countries are modernized now and they are not going to settle for one country being in control of a global currency.
Another obstacle, and likely the biggest one, will be trying to convince countries who have complete control over their own globally popular currency to support a global currency they have no control over that is not tied to any country-specific currency.
The problem with bitcoins as
@Creeper pointed out is there is a finite number available, which is around 21 million bitcoins if I remember correctly. On top of that, a small group of investors own/control a large amount of bitcoins and a lot of bitcoins have been permanently lost through negligence or theft.
I think the best way for a cryptocurrency to become a true global currency is to gain global support and regulation outside the control of a single country.
I think it will also need to provide an unlimited supply of coins (or whatever) over time with daily quotas of newly added/mined coins added to the pool.
As part of the regulations, I think rules will have to be put in place to prevent any single person or entity from owning too much of the currency at one time (it could scale based on percentage of overall pool of course) or else it will simply result in the same wealth-vs-poverty problems the world is already facing with traditional fiat-based currency.
In other words, the best part of the wild world of cryptocurrency we are seeing right now that everyone loves, being borderless and outside the control of countries (at least perceived), are not likely to be traits a future global cryptocurrency will have unfortunately.