Not that I don't agree with you slightly, but what you just described is exactly what they were doing at the end of the first film. They had destroyed most of the forces and were bring in a clean up crew to wipe out the rest and begin take the planets resources.
Now IMO, the sequal should explain that, that was not the only ship in the fleet, and there are hundreds of "mother ships" stroan throughout the galaxy consuming whole worlds as that one did. And now they are sending more ships, or an even larger MS to take out earth and consume her reasources. Also, since we already know what the aliens look like we should see more of their interaction. What type of higher archie do they have? Do they have a queen/king?
But since this movie is not being made this is all hypothetical. I would kill to get my hands on that completed script.