It takes far more $$$ invested to make another sequel (as Civ5 is the fifth in the series) with entirely new game mechanics and features than it would to simply expand upon those that are already there (and were already paid for). Adding features and tweaking mechanics and AI are a lot faster and cheaper to produce and can turn a fine profit (especially considering they have to invest so much less). I wouldn't hold my breath for Civ6, especially if you think they're willing to so quickly give up on Civ5. A more likely scenario (if they did give up on Civ5) is that we wouldn't see a Civ6 at all. A company that has seen multiple expansions and a 4 - 6 year lifespan for each prior iteration for a game isn't going to just shelf their most recent and start in on a new one. Sales figures tell the tale for that one, and Civ5 is a strong performer in the sales category, so, again, you're more likely to see an expansion or two before they start the long (and expensive) process of creating a new Civ.