Bitcoin is already becoming quite unattractive for the criminals, as a lot of them are getting caught while converting the "dirty" coins back to fiat cash. For a while, anonymous coins (such as Monero, Zcash and Deep Onion) were being talked about as the preferred payment method in the dark web, but these coins never made it due to their low liquidity. I guess the criminals will soon go back to their old modes of payment (i.e those using fiat cash). Physical cash remains the most anonymous method of payment even now.
I haven't really had the impression that Bitcoin is becoming unattractive to criminals. Some dumbos indeed have busted themselves by cashing out through KYC exchanges, but a lot of what we see in terms of usage is Bitcoin that continuously gets circulated without the coins being converted to fiat.
The great thing about Bitcoin is that there is always someone who is willing to accept it, which can't be said about altcoins. I'm pretty certain that EVERY cryptocoiner will accept Bitcoin, while with altcoins you'll see that it's quite difficult to get rid of, even when it comes to privacy coins on dark markets. People use the most adopted coin, even when it's lacking in the privacy field.
Physical cash is great for offline payments but sucks balls when you try to transact online. It's pretty much useless there.
