The money would certainly be spent elsewhere, but the point is "why burn precious energy to bring a rock here from China if I can find a rock in my front lawn". Do what you like with the money you save, it is still a win for the planet because you conserved a resource at no cost (if the two rocks are fungible)

Also it occurs to me that while a focus on local may cost gains found in economies of scale, it can net gains in terms of spreading out the burden of negative externalities. This one is murky and not always a clear win one way or the other though.