I am looking at a set of ballistic verbs like nak, phenk 'throw' in a minor Indo Aryan language spoken in Dravidian vicinity, where one verb of the set is reduced to light verb with perfective meaning, occurring often in narratives now. The other verb phenk in the set has successfully replaced nak and occurs in almost every other context, which I am assuming is an effort towards regularization given that neighbouring Dakkani Urdu has only phenk for the verb 'throw'. My question then is, why the grammaticalized perfective aspect marker nak is drifting towards narrative style only? Of course, past tense is used for story-telling but is it just that? Are there other languages where such verbs have completely shifted their use in a specific domain? How do I explain it?
Some languages have a special tense used only or mostly for telling stories, like the Turkish -mış/-miş/-muş/-müş Reported a.k.a. Inferential Past tense (they say ...). Perhaps it's such kind of tense that is arising in the minor Indo Aryan language you are looking at. When a language has two (or more) means to express the same grammatical feature, that state of things cannot last long, either one of them disappears or each of the two acquires a distinct meaning.