At any given moment the price of a crypto coin can be converted into the amount of gold that can be bought at that price. For centuries gold has been used as either a currency or a standard for currency. It is a commodity with actual intrinsic value. While the medium of exchage my have no intrinsic value in itself, it inherits a value from gold. Yes, cryptocurrency really is digital gold.
Despite claims that Bitoin and other coins can rocket to many times their current worth, we see that this cannot happen without the fiat currency drastically dropping in value, i.e., runaway inflation. This actually does happen in countries with economic problems. As a world-wide currency, a crypto coin would be immune to this and gold would keep it controlled.
If a coin reaches success as a currency it ceases to be a potential investment. We will know when that point has been reached when the fluctuations in value are no more than the fluctuations in the price of gold. At the same time we will have determined what the inherited value of the coin actually is. This will not happen over night and the challenge is to make intelligent guesses about when the end point will be reached. I have no good clue at when the end point will be reached, but it appears that we have not reached it yet.