I should note that the cities I'm familiar with, and thus my worldview, have multiple thousands of independent suppliers bringing food into the cities as profitable businesses, not a single altruistic organisation functioning off donations. Therefore there are much fewer catastrophic points of failure - an event that would prevent food from getting into the city would be a large, wide geographical catastrophe. Not the whimsical changing of political positions or sudden misfortune of donors. And in this worldview, when natural pressures such as population overdensity occur, the feedback loop stabilises at a sustainable level - those for whom food becomes too expensive move to cheaper places. I've done it myself.