You're "where did they go?" only works under the false dilemma of 1) survival or 2) extermination.
So, if I follow your thinking correctly — please correct me if I'm wrong — these are the possibilities: European Jews (1) survived in Europe during the war; (2) died in Europe during the war due to causes other than extermination; and (3) died in Europe during the war due to extermination; and (4) left Europe entirely. That about sum it up?
If we accept these four possibilities, and again, I'm not insisting upon this point, then we are still left with a problem that evokes the "Where did they go?" question. I.e., what about those Jews that were sent to the concentration camps in Poland?
We can safely assume that they do not fall into (1), can we not? I.e., we simply have no evidence of their turning up someplace else years later in sufficient numbers to suggest something other than their having died.
I think we can also safely assume that they don't fall into (4). Again, we lack the evidence to conclude that they ended up elsewhere. Nearly seventy years after the fact and twenty years after the fall of the USSR, they haven't ended up anywhere else, and it's worth noting that at least a million of these people were under the age of eighteen, so it's entirely possible that, were they someplace else, they'd still be alive today.
That leaves only the possibility that they died in Europe. Here's where your claim hits the major snag. Because they were removed from their homes against their wills, their blood is on the Nazis' hands. That doesn't prove that they died in gas chambers in an extermination program per se, but it does make them the Nazis' responsibility.
That essentially leaves two issues: The intentionality of their deaths and the numbers.
The intentionality is much of what is debated here, as it includes the matter of method of extermination. It's a bit beyond the purview of this particular post, but I'll briefly state that I think the evidence of gassing is overwhelming, encompassing, as it does, eyewitness testimonies from survivors and perpetrators, documents, and forensic testing, including cyanide testing at Auschwitz and extensive excavations at the Reinhard camps.
That being said, the final issue is numbers.
Of course, your case isn't helped by the fact that estimates of the Jewish population in Europe in the 1930s is even more nebulous than estimates of the Jewish population in the United States today; the Jewish population in Israel showed double digit growth throughout the late 1940s; and Jewish leaders were talking about six million dead before the war was even over--a number that shows remarkable resilience despite the ever decreasing estimates of individual "death" camp death tolls.
Again, most of this stuff has been gone over ad infinitum, but to state briefly: The initial death toll figures that were in the six million range were only calculated when the war was within a few months of ending. By this time, the vast majority of Jews were, in fact, already dead and it was well known that Germany would be losing very soon. These estimates, by the way, were initially then downsized, first by the Anglo-American Commission of Inquiry, which found 5.75 million missing Jews, and then by Reitlinger (4.5 million), who was revised by Hilberg, who upsized to 5.1 million. (Not sure of Poliakov's numbers; I assume Nick could add them). Only with Benz and later writers do we get numbers consistently closer to, or even in excess of, six million.
Jewish population growth in the 1940s was a result of two factors: (1) Legalized immigration, including of Jews who survived the Holocaust; and (2) The deportation or forced emigration of nearly a million Jews from Arab and Muslim countries in the wake of Israel's declaration of independence. With a very small Jewish population to begin with, those factors were going to contribute, and did contribute, to double-digit population growth.
What did NOT happen was thousands, or even hundreds, of Jews claiming they'd been in Treblinka, or Sobibor, or Belzec, or Chelmno, and surviving, despite well kept records recording hundreds of thousands of Jews being sent there. The survivor numbers are in the double digits for Treblinka and Sobibor and single digits for Belzec and Chelmno.
Ergo, if these hundreds of thousands of Jews were not exterminated, as voluminous evidence suggests, then what happened to them?
