Who said anything about drinking from a cup? Not every convenience store has a fountain, and even if they do the performance is inconsistent. Vending machines are definitely not fountains. There's no "cup" a lot of times.
There were many times in my Coke-drinking days when I'd partially empty a 20 oz. I just hated wasting the stuff; but I knew I didn't want to drink all of it. It always went flat before I wanted any more.
BTW, the Mexican cokes are still a bit too big. 12 oz. (355 ml) or half-liter. I find 12 oz., poured over ice and shared with somebody is best; although I can tolerate 12 oz. The half-liter is a disturbing trend. The Mexicans certainly don't need it, since they just surpassed the US in obesity.
BTW, I knew the original coke bottle was smaller and found this article about 6.5 oz. bottles.. Sigh... apparently this was available in the UK not that long ago? Maybe they'll bring it back to the US and finally reverse the trend. The original size was just about right. Yes, I'd pay more per oz., but I'd pay the same per *serving*.
Can the Coke executives get that through their heads? Some of us are desiring a *serving*, not a "most ounces for the buck". Wondering what to do with the excess soda, or being suckered into finishing more than you need... is not a pleasant experience. Having a right-sized glass bottle with real sugar in it, that's what some of us want.