That's easily falsified, because you can trade bitcoin for good, services, and currency which is... things around it. And you can also extort bitcoin from victims. So criminals are not isolated in any way. Their transactions are less visible and trackable. But that's not a benefit. It's the very thing that's permitting them to get organized at scale, and then take on other less anonymous systems. Like topple governments, buy politicians, have their own military like in Mexico, and so on.