>Basically, eventually cash will basically become the “crime mode” for its respective currency. I don’t see why it makes sense to continue to allow cash at that point.

Well, it depends on what the government in question defines as criminal activity. Just because something's made illegal doesn't mean it's immoral, wrong or in any way bad. Many regimes have in the past made all kinds of transactions illegal specifically for the sake of their own criminal persecution of targeted minority groups. This is why argument about "only criminal activity" are both idiotic and naive.