2k's skin is so thick, I cannot believe it. But, as we can see from many people here, they would buy any stuff they churn out to make more money.
Getting more Civ's done does not only require artwork, folks. What about play testing and balancing? Introducing new civs with new UU's and special abilities does require some testing (well, it seems they never tested Civ 5 properly anyway, so I guess there is no need to do it for 2 more Civs).
They are very clever though, releasing the new patch which surely results in 'yeah yeah yeah, the great is so great I can't believe it' hype followed by immediate DLC purchases or so 2k hopes. Smart marketing and it does seem to work.
Civ 5 is getting more and more shameless in my eyes.