The point is that there is massive legitimate use of traditional currencies, which makes them a benefit despite the illegitimate uses. The same is just not true for cryptocurrencies.
Also, traditional currencies have many, many safeguards against illegitimate use. Cryptocurrencies are specifically designed to prevent those safeguards.
There is a good reason that ransomware only took off once cryptocurrencies became available.
It is "legitimate" to use a cryptocurrency to buy pot. It may not be legal, depending on your jurisdiction; but "illegitimate" is a fuzzy word, expressing some kind of subjective disapproval.
I personally think it's illegitimate to criminalise drug use. But that's just a subjective, personal opinion.
What? The peak of ransomwares was during the 2000s.