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.