Here's take. Money is like any good with a price. Limited in supply. Money has advantages over goods. For example, you don't pay sales taxes when exchaning it with other currency, or financial asset. Governments regulate this. So they can make any private currency an asset regulated and taxed by them.
Conclusion to be viable: that it gains international credibility where no sinlge country or collusion of counties can challenge it's viability. Today, that's not the case. Thus, it's just a scarce good (limited supply, with certain properties) at the mercy of dominating economies.
Additionaly, money supply is a tool that goverments or economies need to influence the economy. Since there are strong habits and regulations (for example, it's very hard to reduce salaries under negative inflation, for many legal and cultural reasons) an economic shock under a hard currency (inability to expand or reduce supply in the short/mid term) can trigger a destructive vicious cycle. Like a depression that ruins most of the economy. The economic system is then unable to break that implosion, and the outcome is horrible.
Thus, bitcoins are today a good with some privileges (eg. Not a "good") that may disappear suddenly, and with risks due to that inability to do exactly what we usually don't goverments doing (printing and destroying). Yet, that inability is a double edged sword. And while bitcoins may overcome the influence of governments regulating it, that's very unlikely if you look at what is the GDP of the top 10 countries vs rest is.
So this economist is right. Competing bitcoins is just creating similar assets, and to compete they need to have a tangible (practical superiority) and some strong network effect. For an example of superiority, think of the case of a government that invents a computer that can mine bitcoins at 1000000000 the rate of the rest of the world....then maybe with another cryptocurrency maybe that's not possible. Im am not very familiar with bitcoins, so I use it as an example of weaknesses that might happen.
But all in all, bitcoins are just a virtual scarce good, by definition making supply ever more "costly". But ultimately is like saying you'll trade the Monalisa, and that you can divide that painting in infinitely smaller parts and use it as medium of exchange and value reserve and universaly adopted....besides regulations and law, this is just a scarce good with limited supply. All the variation is mostly demand and the thing that changes is the price per patch of the painting. Which again, has the disadvantage of not being able to provide a stable economy with low inflation and full use or resources, because you don't control supply, and thus, you don't control inflation/deflation.