The reason you are in error with your argument is because you have misunderstood the question. I’m giving you the benefit of the doubt, because another possibility is that you are intentionally misrepresenting it.
The issue does not actually involve capitalistic concepts much at all. The issue is that, so far, there is no evidence that the theory (customers reject food item choices strictly because the portions are too large first, and not as a result of restaurants changing the portion sizes first) has actual documented proof.
The issue is not whether restaurants will respond to changing customer purchasing choices. Of course they will, or they will go out of business.
Let me try this again without some accidental potty mouth.
The issue does indeed involve basic capitalist concepts. If what you’re offering is not what the customers want then the customers leave, period. By focusing entirely on portion size and whether or not it has driven enough costumers away from a given business for them to change you are the one creating the false choice. American tastes are constantly in flux, right now there’s a rebellion against large portion sizes led by the “obesity epidemic” crowd. This really hasn’t been going on long enough for us to know if it’s got enough of the common people support to cause changes. We know it can though, because that’s just a basic concept of capitalism. And we do know, just as the original post said that restaurants stop selling what people stop buying, regardless of the specifics of the item.