Well, I have a very simple answer to your question.  Crypto just like Fiat currencies are created for good purposes.  Let us accept the fact that people can also use them for illegal activities just like child pornography.  If crptos are to be banned because of this, then fiat money must also be banned. But I think, banning a currency does not stop an illegal activity like child pornography.  It could be prevented or at least minmize if governments will just focus on hunting people involved in these illegal activities.