I would assume that as long as people pay then they will continue to update. Especially with so much content not in the game yet :-) I hope they keep going, I'm addicted
ideally it should not need to end ever, as they could keep upgrading to better game engines, better content (360° views of buildings for instance) etc. In 10 years time it could be the game is visually more like Simpsons Hit and Run was and that you can actually move through your own Springfield (view options like Google Earth 3D). It's not science fiction even! :D