a) What on earth does Steam have to do with replayability? Replayability in Civ comes from playing random maps, as opposed to a handful of fixed scenario/campaign maps.
b) There has been no statement from Firaxis or 2K that there will be any DLC for Civ5 other than the initial deluxe offerings (some extra maps and Babylon). But even if there had, what on earth would this have to do with replayability?
I swear, people just take everything they're like and conflate them all together, without actually thinking about what they're saying.
I swear, selling individual civilizations as DLC -- steam only DLC mind you -- actually limits the replayability of CIV. Don't know about you but one of the reasons I go back to CIV IV time and again is to try different civilizations.