if they do pay them for the work before the game is released (they don't hire additional people, they just use the ones they have because they already know the system and it sucks/costs more to train people midcycle), it'll be because that dlc development was made part of the actual game development cycle built around the approval procedures, and not afterwards (the process for staff post release is usually saying goodbye), meaning that the dlc is in fact, a proper part of the game and not just something they decided to add to it later. it means that they're planning these packs out in order to put an additional price tag on the game, and that they didn't intend to end the game proper at where they placed the ending. A true add on that would be worth the money would be an expansion pack, in which by itself is a smaller, separate experience that relates to, but isn't technically a piece of the original game, as opposed to putting characters or side quests or entire zones that you play before the game ends, which becomes a "pay money to fill in content here" deal. If the dlc doesn't work like that and it isn't a graphical mod pack or multiplayer map options thing, then it basically belongs as a part of the game and should be included (like characters in mass effect or dragon age) or doesn't belong near the game to begin with (see adam jensen's "put him on a bus while the story progresses" dlc of deus ex: hr).
at this point it becomes a double edged sword, where you look at it and it's supposed to be valuable enough to charge money for it or else people argue that it should have been included in the first place, but not valuable enough to contribute significantly to the actual gameplay or else people start arguing that it's a pay door. and that's just not good business when people see the 60 dollar pricetag and assume they're getting the full package, instead of having to pay more for a part of the game that was hacked off due to the way the cycle works, and then be told that they have to pay more for it. that's the reason people buy these things for 60 bucks in the first, because they want a whole game (especially for console games where the system is also usually the full package), or else they would go to "free to play" games because at least there you expect to shell out for the stuff they want in it.
you know, it's easy for you to just say "strawman argument" and then expect me to go over your points in order to justify your words, but at this point, i don't think you are going to take anybody seriously when they disagree with you, and when the publishers finally do go too far, it'll be too late to do anything about that particular game, and you better damn hope it's not the game you were looking forward to for that year. when they start telling you that it's "necessary" in order to do something that they either really don't have to do, or do because they think their way is the only correct way of thinking and you buy into it at face value, then you stop expecting anything to get better, and they will feed off of that.
on a different note, at some point we could be seeing other games starting to advertise that they give you the whole package instead of throwing individually wrapped items at you, and then listing parts of the game that could have been packaged as dlc but aren't in order to make it look like they're giving you more for free when it's the same as it was to begin with, and people would be motivated to buy into that kind of thing.