Breslau was Germanys third biggest city in 1945 and a major industrial center now it is called Wroclaw and is in Poland and went from a 98% German population to a 1% German population. The reason the Western Allies couldn't get away with such behavior was the US and their anti-colonial sentiment. By getting rid of the US influence the resulting peace changes a lot. It was the US that blocked French proposals of having the border on the Rhine in 1918.Well, yes.
But then again, going to the left bank of the Rhine is much more than just the Saar. It's reaching up to Kleve and Emmerich in the North, and going all the way down to Karlsruhe. And, yes, the areas annexed to Poland were huge, too. But they were comparatively thinly populated. On the left bank of the Rhine, you have Köln, Aachen, Mönchengladbach, Trier, Saarbrücken, Bonn, Koblenz, Kaiserslautern, Mainz, Ludwigshafen... A whole lot of industry.
The annexations of the territories East of the Oder to Poland and the USSR happened under the oversight of the Red Army, under Stalinist terms. Only very few Germans were allowed to stay. Producing these kinds of policies would be a paradigm change in French political culture, even in spite of everything the Germans had done.
Whereas, if they aren't equally ruthless, then "spontaneous leavings" would be A LOT MORE limited. So limited, in fact, that I doubt that even by TTL's present the regions would even come near to being as Frenchified as Alsace-Lorraine is IOTL.
Not saying it's implausible or anything. Just that it's not really an automatism at all...