So what? The suburb of Heidelberg, Handschuhsheim, is referred to as "Händsche" by the locals. Similar discrepance exists all over the Kurpfalz, Rheinpfalz and other ares. Is it now Danish or what?
Alsatian has a largely different grammar (absence of genitive, only three tenses, no imperative) than German, a different phonology (no "bavarian" diphthong, older diphthong were conserved, palatisation of the long u, etc) and a different vocabulary (french loanwords AND archaisms).
You argue from French standpoint. France has successfully eradicated all major dialects early on in the centralization quest and you think that it would be the same in Germany. In fact, If Alsatian is not German, then the only place where German was spoken in 19th century was the area around Hannover and Göttingen.
No, i argue from an Alsatian standpoint. And German was largely spoke in Prussia in the 19th century, as modern german is basically medieval hochdeutsch spoken by people from Brandenburg. And actually, France is home to far more languages than Germany (Basque, Occitan, Flamand, Alsatian, Breton, Corse, Catalan, Francique, Gallo, Francoprovençal) despite our "evil" centralisation. And i didn't spoke about the Créoles, the Kanaks languages, the Melanesian languages and Tahitian. And most dialects are still spoken (the chtit is the best example).
While some zealous idiot (even more so than the usual Nazi hacks) may have issued a prohibition on speaking Alsatian during Nazi occupation, it would have been impossible to enforce as it takes a very trained ear to differentiate someone from Mulhouse speaking Alsatian from someone from Mülheim speaking Alemanish...
Say that to my grandfather who was in military jail for a few days for speaking Alsatian with a guy from another unit... People from Alemanic regions spoke hochdeutsch on a regular basis but Alsatian learned it after Alsatian (in their family) and French (in school). It was learned as a foreign language. And basically, in Alsace you could be punished as it would have been clearly obvious to german officials, and in the army Alsatian were known to the officers and commissars and were often alone in their units.
I wand also to point something. Serb and Croat, or Russian and Ukrainian are considered different languages. Alsatian is widely not considered a language, mainly due to the opposition of Pangermanists on one side (including protestant priest who refused to use Alsatian as they considered the language of Luther to be the only one worth speaking, it was a big opposition to Alsatian), and of some French Nationalist who wanted to eradicate everything that was not French.
As a wise linguist said :
A shprakh iz a dialekt mit an armey un flot