Note: This is entirely my opinion and research, take everything said with a grain of salt, and do not take anything said as fact.
As we evolve so does our technology and our dependency on that technology, current we're in a so called "age of glass", which is given to this technological era by the glass company Corning to describe and name the current era we live in.
One thing that's still behind however, is the currencies we use, while things like credit and debit cards are becoming more and more used, the existence of paper money is still prevalent, we use it to pay in places like restaurants and other places, while some people still use economical devices such as paper representations such as checks, dollar bills, and coins; We should keep in mind they are simply just that, a representation. Currency has digitally tracked for decades whenever you pay with paper currency, it is translated into digital currency by the Payment Management System the company is using, so why not remove that extra step from the process?
Let's take a look at a graph from 2014, that shows preferred ways of payment.
Shown here the most common payment gateway in 2014 (at least, according to the TSYS consumer survey) is debit and credit cards, with cash being third at only 9 percent. If we were to "inflate" these values and take what 9% of 7 billion is, we'd get roughly 630 million. This is still a very large amount of people in this specific generalization, but not in the grand scheme of things compared to the other 43% and 35% using credit or debit cards.
The main issue with credit and debit cards is cost of production, and cost of usage when it comes to countries with poorer technology, we can look at some statistics on this page and see that roughly 8.4% of 144 countries shown have a 50% or higher adoption rate of credit cards alone, this is not high by any means, but pushing towards a fully digital economy could not only raise this, but allow us to push technology and economy further than it has before.
Lastly, let's talk about crypto currencies as I know they will be brought up when it comes to digital currencies, the issue with cryptocurrencies from an ecological standpoint is that the value flucuates far more often than government ran currencies such as the US dollar, among other reasons including environmental reasons. While I'm not against crypto itself, most cryptocurrencies require mining to verify, validate, and complete transactions, there is some in progress that do not require such methods, but they are no where near as used as say the "Big 3".
I think pushing towards a global, united economy and currency is not only a good idea, but something that should in the future be enforced as inflation and conversion rates in my opinion are one of the biggest flaws of the current non-crypto based currencies, I believe that pushing towards one currency across the globe could at least somewhat saturate inflation to an extent, and would in theory completely irradicate the need for conversion rates, since the currency would be used everywhere, no matter the country.