Honestly, with some developing countries, their local currencies are more or less meaningless, and all important transactions are done through dollars, euro's or renminbi. So, mixing that in with cryptocurrencies is not that much of a strech. As far as it becoming the only currency? I highly doubt it because all serious investments in the country or serious projects being paid are being paid through other major currencies.
I have to say local economies are getting even worse but the big nations are getting a bigger gap each year as well. We had slavery and death and so much misery for the lower class while the upper class had super fun up until 100 years ago or so, then we started to grow a bit better, that crash during late 20's caused things to get a little better, and until around 60's or so things looked nice.
Around early 70's things started to pick up, and by the time 80's came with the cold war and all the gap started to look like there is a growing gap, not a huge one yet but the gap was growing between rich and poor instead of closing. And now we have a huge gap once again, not that we are slaves anymore, but when you can't pay rent, you can't go to the doctors without getting broke, when your cancer costs are as expensive as your house value, then I am sorry but that is not really the freedom we were expecting when slavery abolished, it is really only a whip away from being slavery again.