I don't agree with this view, even in some countries that support BTC today, they have been used to do something illegal.
It's not illegal to mix bitcoins, but it's important to see what people who own bitcoins are doing. It's not about now or the future
I wouldn't say is not illegal to do it now... Some weeks in the news we read articles like:
Prosecutions of Bitcoin Mixers Raise Crypto Privacy Questions
https://www.law.com/newyorklawjournal/2021/05/17/prosecutions-of-bitcoin-mixers-raise-crypto-privacy-questions/?slreturn=20210503103845There they explain how the Mixing process goes against the money laundering laws, and that's why it's considered an illegal activity:
The Financial Crimes Enforcement Network (FinCEN), the enforcement arm of the U.S. Department of Treasury, considers anyone that transmits a virtual currency that is not a party to the transaction to be a “money transmitter” for the purposes of regulation. Based on this framework, FinCEN defines cryptocurrency mixers as money transmitters, which subject mixers to FinCEN’s anti-money laundering (AML), Bank Secrecy Act (BSA) know-your-customer (KYC) checks, and combating the financing of terrorism (CFT) program requirements that have long been applied to financial entities.