To elaborate on my question:
To argue that a good will is the only thing that is good without limitation, Kant must argue that all other good things are not good without limitation. To do this, doesn't he need to assume that these things can indeed be good in some limited ways without a good will? Otherwise it seems that they can't even fit under the category of good things, and therefore Kant cannot move on to arguing that as good things, they are not good without limitation.