I'm sorry, what are you asking?
Some linguists think Luxembourgish is derived from Frankish / Old Franconian, some (e.g. Robert Bruch) think Luxembourgish is Moselle Franconian. Some (e.g. Bruch) think Moselle Franconian is derived from Frankish, some (e.g. Bradley Allan Holtman) consider Moselle Franconian to be West Central High German. It's a mess.
Of course, some of our language categories (e.g. Finno-Ugric vs Uralic) don't follow modern linguistic notions, either, they're just categories of convenience, so I could live with Luxembourgish being under Frankish for convenience, especially with a paragraph at WT:ALB to explain the varying views of the actual linguistic situation.
I'm not sure what you're saying though. Why can't West Central High German be Frankish? I think Frankish is really more of a political term than a linguistic one. There is no definite sound change or other kind of border to mark the change from Frankish to, say, Alemannic. I believe that Frankish is more or less synonymous with what the Romans called w:Istvaeonic, or at least the northern half of it.