It's worse than that. Crypto-so-called-currencies have inherent properties that make them idiotic as a basis for an economy. For instance, the limit, whether hard or practical, on the amount of "coins" along with the inevitability of people losing their wallets means that there will ultimately be a diminishing supply of currency. This leads to a deflationary economy, something any number of experts will agree is a Very Bad Thing.
This leaves the possibility of crypto-tulips as potential investment assets. Again, bad idea. The only "value" they represent is the expectation that someone will be willing to pay more for them at some future time, based on the very same expectation. That can't continue forever. Sooner or later, the supply of new buyers, or shall we say marks, will dry up, and the last to enter the game will be left in the hole, having financed the winnings of everybody who came before them. Simply put, it's a Ponzi scheme, plain and simple.
Looking at it from another angle, the evangelists are promoting "crypto" as both a legitimate replacement for traditional currencies _and_ as investment assets. That combination cannot work. Regardless of any other properties, a workable currency must be stable, with the allowance of moderate inflation. A volatile currency only leads to chaos, historical examples of which are plentiful. Meanwhile, the only way to profit from currency-like instruments (tokens without intrinsic value used to represent actual value) is through fluctuations in exchange rates. The more volatile the currency, the greater the potential gains and, crucially, losses. In the long term, currencies of stable economies largely track one another, provided trade volumes are sufficient. If everybody adopted the same cryptocurrency, that profit avenue would instantly become a dead end.
Most of the hype around crypto-tokens these days centres around their valuation in terms of established currencies, usually USD. Any increase is trumpeted with vigour while downturns are dismissed as momentary setbacks. This very thread has several examples of people boasting about how much their holdings have increased in value. What this fails to recognise is that most of this "value" cannot be converted to real, spendable currency. The exchanges have nowhere near the requisite liquidity, should there be a surge in demand. In other words, actually accessing those gains relies on an influx of buyers. In yet other words, it's a Ponzi scheme.
No matter how things are twisted, the inescapable fact remains that every dollar made in "crypto" today is a dollar taken from someone down the line, someone who will get nothing in return. That, to me, makes the entire endeavor unethical.