I don't see the point: criminals already know what they're doing is illegal. It sounds like a gun shop putting up a sign: "don't rob banks". Innocent people weren't going to do it, and the bad guys already know robbing banks is not allowed.
The current situation neither makes sense. It is the same as saying that it is illegal to use guns unless not bought from gun shops. Alright, so innocent people won't buy guns or will be inconvenienced at buying guns as they will have to look for alternatives, and criminals will
definitely find guns elsewhere.
Gun shops = mixers.
Guns from elsewhere = XMR, coinjoins etc. (which, just between us, are more effective and therefore "dangerous" than the former)