I agree, the system is wonderful, but it definitely seems like it could use a little more drain on it. The AI does run out sometimes but only when they're sending large armies from badly supplied regions (i.e. cities without or with just one farm). Otherwise, they can't run out either, which leads to the endless stream of units unaffected by casualties that I was talking about in another thread.
It becomes even more apparent as you campaign to the south and especially to the east. The Black Sea region is rich in farmlands and the Persian territories are literally swimming in food (cities with 3-5 farms are pretty common there). For instance I was bold enough to conquer the Troad (northwestern tip of Anatolia) before finishing my campaign to take over everything to the northeast and that left me under non-stop attacks by the Persians. By non-stop I do mean a constant flow - coming in armies of 6-8 brigades, by the time I wiped them out, the next army of 6-8 was already on its way. This began from the moment I set foot on Anatolia and continued all throughout my Black Sea campaign.
Let me paste in my related comment from the thread I mentioned:
"Though I still think some tweaks to the food usage could have their merits. Especially if there's going to be a new stat to limit recruitment - food is already in abundance with enormous surplus building up in cities behind the front and if recruitment related consumption drops, the surplus will grow even larger.
Even now, food supply is nearing the point of irrelevance on the grand scale. Of course, refilling your newly acquired city can be challenging at times, and as such, a lot of fun too, but it's not an issue of supply, it's an issue of transportation (which could also be improved as suggested by Escipio's great post earlier). As supply is infinite already. Which also puts a small dent into my sense of napoleonic glory - to zoom out and see many of my cities just a few nodes away from the front sitting on food reserves of over 10,000 (some at even 20-30,000) makes me feel like my conquest of the known world is pretty much inevitable, it's just a matter of time, because there's no way I can screw it up, no way to run dry by overextending myself, no amount of defeats can stop me, for I am Maceborg, all resistance is futile! So yes, I think there's room for increasing the warmachine's drain on ye olde bread and butter here. ;)"
I think the solution would be to improve the transportation, so that a greater share of empire-wide food production is utilized on the front (no more insane reserves) and then increase the food related expenses (or decrease food production rates) to adjust to this increased capacity.