That's one possible benefit of a townhall. I remember that Civ2 had buildings that reduced the city's upkeep costs and different buildings that produced gold.
If they went with your idea, townhalls could cancel out the upkeep cost of towers, arcane libraries, etc. If a town was budget neutral, marketplaces and the like would help to make the place profitable. Of course, you'd build a mix of both since you're limited by city size. I think it would lead to smarter building strategies.