Cash is already phased out. Take $12,000 out of your bank. Oh, that's right. You can't. They don't have that much. You have to call ahead.
That may be true for law abiding citizens, but consider the drug trade. The people who grow opium in Afgahanistan or refine heroin in Columbia want to get paid from the cash sales to users in the U.S. and Europe. That mean a net outflow of funds from industrialized nations to third world countries to pay for the drugs. All international terrorists have to do is pay the suppliers in Afghanistan, Columbia etc. and have their agents collect cash from dealer networks inside the U.S. and Europe, maybe offering a discount relative to other ways of getting cash out. The cash itself does not have to move far at all, certainly not across borders. Given the huge scale of international drug trafficking, only a tiny diversion of its cash flow is required to keep local terrorist cells comfortably in business.