Indefinite bare classifier-noun phrase structures (CL-NPs) in modern Southwestern Mandarin (SWM) are derived from the impoverishment of ONE in morphosyntax: the formal features of ONE get deleted if the Num node hosting ONE is structurally adjacent to a classifier. Historically, however, the absence of the numeral 'one' in CL-NPs used to be a low-level PF phenomenon in earlier SWM: the numeral is inserted, but then gets dropped at PF. Conceptually, there are good reasons to suggest that morphosyntactic changes are secondary to phonological changes (or more generally, interface changes), if the mechanisms of language change are considered seriously. The empirical discussion regarding the development of ONE-impoverishment in SWM strongly supports this idea: a morphosyntactic innovation may be the result of a phonological innovation, but the inverse should not be possible.