>Who would substitute cocaine with coca tea?
There are really two things to respond to here: first, all powder cocaine starts as the leaf, so the powder cocaine market would move downstream to extraction from widely-available coca leaf. This is the biggest reason we haven't tried this already. However, I hope that the economics will not favor transporting large amounts of still-illegal cocaine across long distances, so that instead it will mostly be produced near the point of consumption, with lower profit margins and less violence.
Second, addicts are more likely than you think to recognize their own usage as a problem, yet rely on the drug to do things that most people expect to be able to do. Someone who uses cocaine so they can focus on work or participate in social activities would probably be willing to substitute at least some of their usage with tea, if it were cheaper than powder and legal. Drug abuse isn't usually a problem of simple uncontained hedonism. It's not a perfect analogy, but the availability of beer probably likewise moderates the demand for spirits.