How does the language of the Franks, which is known more or less from place names and personal names only plus a few borrowed words, tell us anything about their ethnicity? We knew they were Germanic before that. Are you implying that there is a connection between Dutch and Frankish "ethnicity"? That connection, though it is probably there, is not very direct over a thousand five hundred years...
I don't know why the fact that a Frankish-derived language (Dutch) being spoken in a certain region (the Netherlands) should force us to believe that the Franks were originally from there. Of course the two facts are related, but the fact of the language being spoken there does not itself tell us that at one point in time its inhabitants were primarily Franks any more than the fact that a Latin-derived language is spoken in Belgium tells us that at one point in time its inhabitants were primarily Romans. Again, I understand the connection, but it is not made on linguistic analysis alone. This is why most books/articles on the Franks that I've read (Wallace-Hadrill, Bachrach) don't mention Dutch. Your comment about Franci seniores (what does the Franci superiores have to do with anything?) does nothing to explain to me the importance of their language. Their well-attested written language was Latin. Why not mention that? In fact, it tells us far more about the Franks than that they spoke a proto-Dutch language. 
As to the remarks concerning the pertinence of my comments only for the Merovingian period, I was not aware that the article was about the development of a language and not about the Franks, a term which does not have a consistent universal ethnic/tribal meaning by the end of the 9th century (and probably by the end of the 8th). 
In short, I don't see how you have actually answered the objection. I don't know what "German" point of view you think I have, but I find that ridiculous. And shouldn't a minority of scholarship moreso undermine your "Dutch" point of view? Finally, when did I declare myself a Carolingian specialist? I have no specialty.