Well... without knowing anything about the modability of Civ 5 and others, I do have some insights in the modding community. And when the community has shown time after time is that enough people play a game, and love it, the possibilities are endless. Comming from a long period of playing Skyrim and seeing what the modders have been able to do with the game is amazing. Custom DLLs, hooks into the engine, extending and adding new code making new stuff possible that was deemed impossible, new worlds bigger then the game itself etc, new animations not thought possible with the gaming client etc.
So VP devs might make a final version, polish it and release it and then that's it. But new people will play Civ5 and they will start something we never thought of. Or they wont.. and the main reason being that the fanbase is too small and the time required to blow our minds is too steep, the knowledge needed too high etc.
And if the fanbase is too small, then the game is slowly dying anyways.