What surprises me is that they didn't realize that SimCity doesn't have the right game mechanics for classic DLC.

Instead of going the DLC route, they should have gone the user-gernerated content route. Allowing users to not only share and rank their cities but also to create additional content (3D models, conversions) and to interact with the game mechanics using an in-game scripting language.

By that last part I mean things like programming the logic of the in-game traffic lights, train schedules, and so on.

Pure, raw, script-based micro-management.

Had they managed to implement a game incorporating content-sharing, they might have seen the sales of their core game skyrocket, fueled by the positive marketing feedback of a content-generating community.