The problem with the assumption of the economist is that he creates all this mental exercise from a false premise, he says that if bitcoin was used just like any other currency it will be worth 20 dollars, and there are two flaws with that kind of thinking, the first one is since when currencies are only used as currencies and not as a source of speculation? Look at the Forex market, and second bitcoin is in no way or form an ordinary currency so all what he is talking about can be disproved with just those two points.
Well, these two points of yours don't actually disprove the authors' argument. Because it is not a false premise, it is just an assumption, basically "what if". You may accept it or you may discard it. In the first case, you could then argue what Bitcoin price would be. Though it would be hard to say since we don't know and can't correctly assess what Bitcoin adoption might be and what technologies like LN and its likes would be used by now.
From wikipedia
https://en.wikipedia.org/wiki/False_premise'A false premise is an incorrect proposition that forms the basis of an argument or syllogism. Since the premise (proposition, or assumption) is not correct, the conclusion drawn may be in error.'
It is a false premise that currencies are only used as such when we know that no major currency follows that pattern, every major currency is used to speculate with the strength of the economy of the country or with other factors, the scenario pictured could be correct but it does not change the fact the premise is false and as such it is never going to happen and the conclusion is mistaken.