Never upgrading a market is a bit extreme if you do the maths. The market chain food costs is capped at 2 for a structure that adds (at the top end) 2000 to wealth from commerce, or, at 25% tax rate, 500 per turn, in addition to increasing town growth. If you took the 2 food and added that to town growth for a 10 province empire, getting an extra 500 per turn is going to take 2000 increase in town wealth, or 200 per province, which is 100 turns. Not exactly the best use of food, especially since we're discounting town wealth bonus from the market chain.