Everything is used by criminals, not just Bitcoin. For sure, the great majority of the criminals are using fiat. The more dangerous ones are definitely using fiat. They are the ones who are wearing suits and are occupying high positions in the governments of different countries and in the top executive positions of various international companies. In the low level, even scissors and frying pans and pins are used by criminals as well.
Yes, I agree. No one can control crimininals of using different currencies. All currencies can be a useful tool for them in doing crimes. Not to focus only on bitcoin because mostly used are paper money and not bitcoin.