Earlier quoted context omitted.
> Full-blown lexical tone, at least in some cases, seems to develop as the result of reinterpreting tonal correlates of some other phenomenon I would have said that the "I don't know" sequence matches this description -- it's just the tonal contour of the original phrase, with the other details stripped out. I don't see how that differs from Punjabi syllables preserving their (originally) allophonic tone while losing…
> it's just the tonal contour of the original phrase, with the other details stripped out. I don't see how that differs from Punjabi syllables preserving their (originally) allophonic tone while losing their (originally) phonemic aspiration, such that the tone becomes phonemic. because, as you elaborate, the English example you mention is really isolated - it's not enough to spawn a system of tone. Even in this case…
It's not contrastive in that there is no minimal pair that would demonstrate it. But good luck being understood if you don't produce it correctly. The lesson I would draw here is more "minimal pairs aren't sufficient to demonstrate every phoneme" than "it's not a phoneme until a minimal pair exists".
> the English example you mention is really isolated - it's not enough to spawn a system of tone
I didn't claim and don't believe that English is developing lexical tones. But I do believe that the development of lexical tone in this single unusual word is fundamentally similar to the larger-scale development of tone in other languages, and that the still-obvious relationship between the phrase and the tone sequence makes this a good example to help English speakers understand how it can happen.
> And of course there's more than just question-associated tone, there's also focus-associated tonal prosody.
Very true, but much harder to describe.