The large number of overweight Americans is a huge pool of excess food and takes a lot of extra daily calories to maintain. As is the large number of livestock being produced and all that corn ethanol etc.
The top of the curve removes that as we can't pay for the inefficiency's of livestock or the extra daily calories to be overweight and feed 50+ billion people.
"carrying capacity" is set by a lot more factors than just food.
Human population is expected to level off at 10-13 billion based on factors relating to social organization, not the 50+ billion that could theoretically be fed.
Replace food with water, land, etc and you get the same instabilitys. Social factors at the limits can easily lead to war, revolution, and or the breakdown of society.
sure, but you don't need to be at the limit to have that sort of instability. It becomes a bit more pronounced when there's less leeway, but it's not different in kind, only a little bit in magnitude.
With humanity, "carrying capacity" is unlikely to be food-limited.