I don't think we need to worry about people that abandon the game after a year or two, mods are made mainly for dedicated fans who stay with it for longer. You know, Civ3 is still being modded
And mods aren't made for money, so it's not that important how many people download them, but of course a solid user base is important to test the mod, provide feedback and ideas, or even join the development team.
An expansion is possible, as 2K Greg mentioned that "there are some much bigger plans we hope to announce in the coming months".

An expansion is possible, as 2K Greg mentioned that "there are some much bigger plans we hope to announce in the coming months".