> But to assume that they would buy more of the exact product that caused the problem is really silly. Why not buy some of all the other products in the world that would make me better off, and one that addresses my particular problem?

I don't understand why it is even remotely silly. Consider alcohol and tobacco, or any other addictive substance. A very common dynamic is that the addiction feeds itself. Sugar might be less addictive and/or less unhealthy than alcohol and tobacco, but it's still addictive and unhealthy.

While it might be possible that the more extreme statement that most cola-drinking obese and diabetic people keep drinking more and more cola is false, 'buying more of the thing that is the problem' is not at all silly. And anecdotally I find that obese people do tend to get stuck in this spiral and consume massive quantities of sugar water: that's why they got obese in the first place!