The realization came when I checked how a dictionary classifies the word instead (specifically, I was thinking of the combination instead of). Now, in Russian, it would have clearly fallen under the category of prepositions, because it would form a semantic relationship with a noun, and because it would syntactically govern the morphological form of that noun.
In English, of course, there is no morphological test for syntactic government; and semantics somehow didn't quite count, because the dictionary judged instead to be an adverb. Which I would also have done in a sentence like "take X instead"; but would have failed to do in "take X instead of Y". In Russian this "instead of" would have made a compound preposition — like all those в течение, по поводу, независимо от, etc.; but the English grammar apparently follows a different logic.