It was only five short years ago that the majority of bitcoin transactions involved a criminal element. That number has now dropped to less than 10% of BTC transaction. This was revealed in a recent Bloomberg interview with a DEA official.
I do not know how these studies were done, and how criminal elements were identified? But I totally agree that the rate of bad use has fallen.
I think the main reason for this shift is the encouraging heights 'bitcoin up to 19000" and the positive news "Media effect".
The other thing is how will legislation be prepared for all cryptocurrencies since many of it have been used for dark activities and money laundering? The standards of financial institutions also lose the structure of decentralization.
A 90% drop in the illicit use of Bitcoin will have an immense impact on lawmakers and open up those who might have been previously for an all-out crypto ban.
Even if these numbers are true, It will not has an impact on lawmakers, it just means that "Bitcoin is not bad".