As for hurting modders, how many modders would make a mod for an expansion pack rather than the vanilla game and incorporate those units? It's essentially the same thing.
It's not even close to the same thing. A expansion pack is
playable, and contains entire new mechanisms not present in the core game. Expansions will add better espionage, add corporations, that sort of thing; no amount of custom content can make up for that. The DLCs we've seen so far don't come close to that, so using past games' expansions as an example is just a flawed analogy. These factors make an expansion product desirable to people who have no interest in using mods or buying DLCs; they'll buy Warlords, or Beyond the Sword, because of the way they change the underlying game, and use them for quite a while before they get tired of it (if ever). Sure, the expansion will also provide a bit of extra content for the modders to play with, but that's not their primary purpose.
This is completely different from buying a content pack solely for the purpose of playing someone else's mod (or creating your own mod) using that content. There's no guarantee of a return on the time invested, and even the addition of a scenario or two wouldn't be enough for many (most?) people. And again,
what happens if no one makes that mod? The OP claimed that if they were to sell a Mad Max-style pack that he'd make a mod, but
what if he didn't? (Or what if he did, and it just sucked because he's not nearly as good of a modder as he thought he was?) Who, in their right mind, would pay even $5 to gain that content without a guarantee that they'd get an enjoyable game experience out of it? Or more specifically, who would pay any amount for it when it's first released, in the possibly vain hope that three or four months LATER there'd be a good mod that used that content?
Most of the reskins people have done so far for Civ5 (the closest analogue to this idea) have centered around importing already-made Civ4 assets into Civ5's formats, or modifying a simple 2D texture without adjusting the mesh underneath. This is FAR less work than creating an entirely new model. So if you're creating new content for modders to use, it wouldn't be nearly as a small amount of time invested as those reskins took. More time spent means more money spent on labor, but do you think there's even a small chance that enough modders would buy this pack to even come close to breaking even on those costs?
And that doesn't even begin to cover things like copyrights; modders can use things like Mad Max in our mods because Fair Use is pretty generous when you're not making a profit (although that's not the only factor), but Firaxis couldn't use that sort of content in its own releases without paying royalties. So anything we'd get from them would, by its very nature, be watered down with tons of "generic" units... unless they were basing it on a property Firaxis already owned, like Colonization (which they did in Civ4) or Alpha Centauri. And if they're going to create Alpha Centauri units, then why wouldn't they just make an offshoot game using Civ5's engine, just like they did for Colonization in Civ4?