Extinction with the last native speaker's death seems a reasonable yardstick to me (and yes I know the Vatican still uses Latin). After all I have been known to say nggak iso from time to time, but that doesn't mean I can speak Javanese. I assume your great-grandmother's expression means roughly "Be off with you, you daft ninny", which my grandmother (from the south of Scotland) used to say from time to time, so it would seem to be more from the English spoken in Cornwall rather than from the Celtic Cornish tongue.

Bingley

Bingley