> Are we not heading to the point that, for a nearest neighbour
> approach, we have to allow for auditory closeness as well as
> articulatory closeness?
Definitely. An important and often crucial part of any sound change is
the speaker-to-hearer link (how else could anything be
re-phonemicised?). It's naive to imagine that sound change takes place
in the mouth of the idealised "average speaker" and so to insist that it
ought to be articulatorily continuous.