We're talking about different classes of criminals when discussing fiat and cryptocurrency transactions. The local drug dealer isn't inclined to accept cryptocurrency because they are running a cash business - they buy in cash, they receive cash and they restock in cash. Besides that, their customer base is unlikely to be super sophisticated to use Bitcoin, or at least enough customers don't do it to make it worth additional hassle. The type of criminals you see talked about in the news and referring to crypto are large scale - randomware attackers, money launderers and criminal syndicates that generally just need to move money across borders with ease.
Drug dealers need something tangible that can be used offline in the easiest way possible. Cash fits the bill nicely due to its wide acceptance by business and merchants alike. I doubt drug dealers would want to get into something as transparent and volatile as Bitcoin. Not to mention, it requires some level of technical knowledge to get used to. With Fiat (cash) you just hand in the money to the merchant and that's it.
For these and many other reasons, cryptocurrencies like Bitcoin and Ethereum won't attract many criminals into them. Only hackers and scammers will find crypto useful for their own needs. As long as the majority uses crypto the right way, there should be nothing to worry about. Just my thoughts
