any other change that would result in it(rapid and massive change in Diet across all of America, sudden loss of access to beef, massive spike in wealth allowing higher quality eating options, widespread death among people eating beef meals) would leave such a massive, noticeable trail that it is a useful metric for the current system. I mean, if enough Americans went vegan that there was a massive dip in Big Mac sales, that would cause massive sweeping changes in how food is produced and priced, as well as the lifestyle of the average american. Anything less than that and it's just the current system failing.