On its face, this kind of makes sense. If there don't exist jetpacks, I don't want one, and the object has no affect on my happiness, but once someone invents one, there is the possibility of it making me happier by getting one; Galbraith argues that we're just as well off if the jetpacks had never been invented.
I think that Galbraith has this all wrong though; in my mind, we do not desire specific items, we want items that are of a specified use to us. Every thing belongs to a family of goods/services that we desire regardless of specific items within that family that exists. We have a desire for transportation, entertainment, education, leisure, human interaction, etc. Advances in technology, and indeed, affluence, provide us with better, more efficient ways of satisfying these desires. If I want to see the world, and it happens to be 1750, then, unless I happen to be born into extreme wealth I will have trouble fulfilling that desire; I might be able to work my whole life for a trip to Paris, and if I live in a small town even a trip to the nearest large city will be an extreme cost in time and money. Affluence allows us to explore these dreams at a much lower cost than ever before. While I would have never yearned for a car before one was invented, I am nevertheless better off after I get one than I was before.
I think back to Alasdair MacIntyre, who claims that the goal of a human life should be to build a positive narrative; it seems to me that affluence allows us to build a richer, deeper narrative, and gives more people the opportunity to explore things they never before would have been able to do. Sometimes people don't use their wealth to build their own narrative, but that is not the fault of wealth.