Most of Carinthia became German by the end of the high middle ages and so was Klagenfurt by the 19th century. Whatever happened prior is frankly meaningless, you could use the Polabian rebellion as an argument as to why the Germans would never conquer or assimilate the Polabians and yet here we are.
If Hungarians managed to assimilate the locals so thoroughly without having big numbers of settlers so can Germans. Also why use Slovenia as an example and not say East Germany, the Sudetes, Silesia, Pommerania, Prussia? Pannonia was hardly a particularly densely populated or entrenched region, there is a reason why Slavs survived outside of it and not inside it.
Idk if "by the end high middle ages" is correct assesment, "by the end of late middle ages"is probably a more correct one. The difference is, the things which I talk about happened in situation of German military victory, not German military loss (as in Polabia). If Germans, out of their good will, gave Slavic dukes overlordship over that region, why do you think they'd be any more effective later?
Especially as we know what happened to Germany itself later and the fact that march was at least temporarily lost to Moravians and probably Hungarian invasions enabled Germans to reconquer it.
Well, Pannonia was a centre of Hungarian state, while it won't be centre of German state (and still it took centuries to completely assimilate Slavs in Pannonia). And I didn't say assimilation among Germans would be impossible, just that it'd be not particularly likely.
And a fair deal of Slavs left Pannonia when Hungarians invaded, don't forget about it.
If you want to analyze these examples that's fair:
a) East Germany - you should treat Lusatia and non-Lusatia separately, as in Lusatia, Slavs survived for very long, and in relict form (Sorbians) they're still around now and what screwed them the most was frankly 30-years war and XIXth century, had Bohemia retained it's independence and Lusatia (it was part of Bohemia for a long time), you'd have funny speaking Czechs, not Germanized region
As far as non-Lusatia is involved, Germans had to replace a great deal of local population to even be able to rule the place, Polabian Slavs were just that unruly, and still - in Middle Ages speaking "Wendish" was prohibited in the cities, so the speakers obviously were there. And the connection between Polabian identity and paganism was also meaningful, as it faded away along with Christianization. 30-years war also made it's impact there. Still Slavs survived in
https://en.wikipedia.org/wiki/Wendland up until XVIIIth century.
b) in Pommerania adoption of German culture by the upper classes was more or less voluntary and still, Slavic speakers existed there by all of XVIIth, XVIIIth, XIXth centuries and only state germanization programme (in XIXth century) made them go extinct.
Things like nationalism and stuff are entirely unpredictable with POD in IXth century, don't you think?
c) Silesia - in Silesia most of germanization was an effect of state germanization programme connected with so called "Prussian model" of ending serfdom which caused huge migration of dispossesed peasants to cities, when they tended to lose their previous language.
But even in early XIXth century, about a half of Lower Silesia was speaking "Wasserpolak" -
https://en.wikipedia.org/wiki/Wasserpolak, and Upper Silesia was not assimilated at all.
d) Sudetes - the region was even more sparsely populated than Pannonia prior to German settlement, the map with Czech proto-urban centres attached to Dusan's Trestik's "Rise of Great Moravia" shows zero of them in Sudete region.
e) Prussia - idk why you even brought that example up, native population of Prussia (Old Prussians) was Baltic, not Slavic - Slavs arrived there as migrants, invited by Teutonic Order (so called "Masurians") and they basically retained their language up to XXth century, but considered themselves Germans due to being Lutheran and being ruled by Prussian dynasts.