Well, Develop a product based on how well you want it to sell. I don't think WotC has any fantasies about Gamma World replacing D&D.
I imagine that each person will have an average of one GW campaign happening in their "gaming circle". That's enough for WotC to make good money selling the books. But that's not enough density for regular card trading between friends or a thriving local secondary market.
Sure I doubt they think it will replace D&D as the primary game... (They even mentioned the picture it as an in-between thing.) But I also don't think they're designing it with the mindset that "no one will buy this game."
Did they know MtG would be a raving success when it was released?
Do business with optimism- and optimism says people will like/want your product, and when people want your product they find a way to use it.
In this case, if that means trading cards- they'll find a way to trade them. (Even if it's mainly on-line.)
Hell I used to collect/trade with my friends some of the weirdest collectible cards... (Anyone remember those dinosaur invasion cards? The ones where some weird quantum fluke caused our world to merge with a dinosaur world?)
My real guess though is they're designing it with the idea that the cards are a sort of tag on purchase. You're in the store, buying books.. at the counter... oh hell why not just toss in a pack or two of Gamma World cards! They're only 3 bux!
Just like the game is supposed to be a sort of "break" from your normal D&D game, the cards are just designed to be an impulse buy.
Unlike MtG they don't really seem to be designed with the idea that you NEED certain cards. You might want them, but you're not really going to suffer much without them.
They're at that price point where compared to the 30 dollar book in my hand, the 3 extra bux aren't going to break the bank... and if they're not as great as they could be? Eh... I spent more on my coffee on the way to the store.
I see it as a win-win situation myself.
As you point out, I don't think the game will garner enough support to warrant a whole product line of published books supporting it, and as I said earlier I think cards allow them an alternative to give us more support without it being out of the price range.