I brought our discussion to John McWhorter's attention. He replied as
> Dennis --
> What fun; thanks for bringing me in.
> From what I see, the verdict is this:
> Periphrastic DO was optional but meaningless in Brythonic Celtic,
> but in non-English Germanic, had a pragmatic meaning, i.e. was not
> What happened in English was that the optional, meaningless DO
> became obligatory and meaningless. That is an unexceptional
> reinterpretation amidst language contact -- what is optional in one
> language becomes obligatory (simpler) in the receiving language.
> The question is why, if English's DO is just a Germanic inheritance,
> it is now obligatory and meaningless when it is neither of those in
> Germanic other than in English.
> Great to see anyone actually engaging in the details of the
> argumentation ...