Stalin did not want the Germans to be united in one, rearmed, bitterly anticommunist nation--which is what expulsion of all East Germans to the West would have meant.
Yes, the inital expulsions would temporarily cause disruption in the West. (It would also guarantee a violently hostile West--something that Stalin did not want in 1945. He knew that the West would not object too strongly to expulsion of Germans from, say, Czechoslovakia, because of the argument that they had been a "fifth column" there. Not to mention that it was not the Communists but the "democrat" Benes who had taken the lead in demanding the expulaion of the Germans from Czechoslovakia.) But in any but the shortest run, the addition of almost 20 million people to West Germany enormously strengthens its industrial and military capacity--and even in the short run it has the disadvantage of immensely strengthening the Right in western Germany's politics (against the Social Democrats who advocated a policy of neutrality) and making it much more likely that West Germany will rearm (with American encouragement). Of those twenty million people expelled, many will become soldiers, or will beget soldiers (or workers in military-related industries). This is *not* what Stalin wants--especially when he can instead have those twenty million industrious East Germans working for *him.*
An important function of the GDR, incidentally, was to give the Soviet bloc a bargaining card in negotiations with West Germany--we'll be nicer with your fellow Germans in the GDR if you'll do this or that for us. Why give up that card?
Oh, and what happens to West Berlin in this scenario?
All correct considerations. But I'd add that the West would simply not stand this. They wouldn't say, oh, let's welcome those 20 millions so that we will have a more populous, stronger, and rabidly anti-Communist Western Germany. No, they'll tell Stalin: stop that.
Now, in 1945-1951, it's not that the West couldn't defeat the Soviet Union; they had nukes, Stalin had none (save a couple of tests in 1950-51). It's that there wasn't a good enough reason to go to war - again, even if lots of right-wingers said that that was the right time to do so. The populations wouldn't stomach it.
But this scenario would provide the reason.
Naturally there would still be strong opposition to war in the West, by left-wingers. Yet Stalin knows all the above, and while lots of bad things could be said about him, he wasn't a risk-taker. It will never come to the West having to threaten him about this idea, because he'll never even consider trying it.
Nor are any of the deportation scenarios any indication that he would. In all cases, a justification could be produced. Some minorities within the Soviet Union were collaborating with the enemy, during the war, it was wartime, and it was an internal affair of the Soviet Union. Russians moving to the Baltics or in Central Asia? That's spontaneous internal emigration, and again an internal affair. Germans out of Poland and Czechoslovakia and into Eastern Germany? It's the Germans who actually want to go away and the authorities of Eastern Germany welcome them. Poles out of Ukraine and into the new Polish borders? Ditto, Poles wanting to live in Poland and Poland welcoming them. None of the deportees could later claim otherwise, because, to whom would they be complaining, in Communist Eastern Germany and Poland?
But not in this case. Sure,
some young, unattached, anti-Communist Eastern Germans might welcome the opportunity. But the vast majority of any people, even if faced by an oppressive regime, want to hang on to their home, their town, their job, their life. They'd be forcibly deported. Nor would the authorities of the place they're sent to be happy to welcome them, not in the millions, not to become accomplices in the largest mass deportation in that time frame. And there would be a free environment to air their complaints.
So it's ASBs.