I would concur that the DLC isn't intended to fix flaws in the game -- if it is, I don't think it has succeeded. However, I disagree with your assertion it isn't intended to extract more money from paying customers.
If the publisher were truly interested in adding value, one would think they would keep working until they resolve the flaws in the game. In my estimation, only a tiny minority of users are satisfied with the game's AI ... in games that focus on multiplayer, such a lackluster AI could be excused, but Civ V -- despite multiplayer capacity -- remains, as with its predecessors, a single-player game. The AI is still weak though it has gotten a little less bad, but compared with, say, Stardock, Firaxis has been very slow with patches and they seem to still be a long way from having a competent AI. None of the DLC addresses the flaw issue, and it feels a lot like Firaxis doesn't believe there is anything wrong with the AI nor other issues with the game and only a minority of players are concerned with it. So long as customers keep forking over for DLC, I don't think they really have any reason to care.
I don't buy DLC for the game because I don't feel I've yet gotten my money's worth for buying the game to begin with, and paying more money for "content" that doesn't fix what I perceive as enjoyment-diminishing flaws would only make me as disappointed in myself for supporting what I consider bad customer service as I am in the game.
As for supporting a "secondhand market" ... what secondhand market? So far as I know, the Terms of Service for Civ V make it illegal to resell your game; at least, I believe that is the case with the digital downloads. If you are dissatisfied with the game and want to pawn it to get at least a little of your money back, you're stuck ... I cannot think of a single motivation for a software publisher to do this except "haha, gotcha, don't like it? tough! we got your money and you won't get it back!" They get away with selling a product under such terms that they have no motivation whatsoever to make a game right, by taking away their customer's rights to resell their purchase if they are dissatisfied.