I guess that I would be the person on this forum who most believes that this technology will be adopted(?). I am not saying that there is no chance that I am wrong - there is always a chance. I believe that there is a high probability that this technology will be common and mainstream in 10 to 15 years.
Most countries will indeed probably have there own cryptocurrency but that won't necessitate making others illegal. There are companies which already specialize in trading between cryptocurrencies and it will only get easier and cheaper. There are already cryptocurrencies that are pegged to specific national currencies. The potential is just so phenomenal.
As time passes people become more open and apt to adopt new technologies. Twenty years ago there were many people who wouldn't even use a bank card. Twenty years before that, there weren't any bank cards. Now some people never carry cash - that's electronic currency.
For a broader interest on adopting technology, check this out:
https://ourworldindata.org/technology-adoption
There is a chart just a short scroll down that is quite informative.