I like the way you think, though they are making a game that caters to a wide variety of players. There's SP, there's MP coop, there are private servers, there are really big servers with dozens of simultaneous players, there's PvP. How do you build a system that's a good compromise for all these different playstyles? Mostly they've made the game really easy to mod. In general diminishing returns is a good concept, but endgame suffers in that regard. * space*
Anyway there are strong benefits to having infinite income (and an infinite way to dispose of said infinite income, infinitely) from both zombies and cities, not from a realistic standpoint but from an MMO standpoint.