You make a good point about the cost of adding liquor to the coffee (I have done that myself as well).

Another item was the waste factor -- people would get a cup, try it, send it back, and order something else. So they were paying for the FreshPoo and another beverage, while throwing the FreshPoo down the drain.

Plus, as you point out, the soft drink or other beverage is very likely more expensive than the coffee, so each cup of FreshPoo that was was returned costs them even more.

On morning DL flights, I usually drink coffee. On morning UA flights, I usually drink cocktails. That's a cost difference right there.