I agree bitcoin having a great potential on being a world currency, but replacing dollars is way too far, 15 years is not enough, also even if bitcoin has a a good price in the future, it is still not enough. It has a great volatility that will affect a country's economy. Also, you can't expect that all people will be supporting or using bitcoin and many other digital currency.
That is certainly true, mate. Not even with all the strides that Bitcoin has been making over the course of the year, will make everyone from around the world to use it. With its instability in price, it's good for profiting and investing, but not ideal to serve its purpose as a serious currency, especially for a country which may limit its ability to prosper economically.
Therefore, Bitcoin needs more time to mature and become a solid cryptocurrency with a large user base. Even then, it won't be able to replace fiat currencies such as the dollar, since the latter is backed by a government and used as legal tender.
But, with all the trends that blockchain technology has experienced lately, you could expect to see digital fiat currencies issued by governments sometime in the future. Just my thoughts