The easiest way to explain the weakness of the Madagascar Plan is to simply note that, as a first step, it required a successful Operation SeeLowe. The plan was dependent on full access to the British merchant fleet, as well as those of the other occupied countries (aka: the same ships that were being sent to the bottom in the Battle of the Atlantic) and was designed to max out at 4,000,000 deportees (1M a year./. There were vastly more than 4M Jews in Europe, with the generally accepted figure being 9-9.5M.
This sort of scenario also ignores the Reich's other crusades against minorities and nationalities, including the Roma and the Slavs. Depending on whose figures you accept the Reich killed between 300,000 and 1,000,000 Roma and 2.1M non-Jewish Poles, plus hundreds of thousands of Salvs in the rest of occupied Europe. The straight up extermination camps killed around 3.1M people, mainly Jews. The other roughly 6M civilians (around half of who were not Jewish) who were willfully murdered died due to salve labor, shootings, hangings, being thrown off cliff en masse, drowned by being thrown off bridges or cliffs into rivers/lakes en masse, etc.
Most of these were "quietly buried", burned in open pits or simply left to decompose. The problem is that a burial pit with 50,000 bodies can't really be hidden.
Had the Reich been successful the goal was to KILL 85% of every Slav in Poland (that is 23.5 MILLION Slavs) down to 65% in European Russia, with figures for the rest of Eastern Europe falling between these two figures.
The Reich was evil incarnate. Its written plans called for the murder of tens of millions of people. Had the Nazis been successful the Six Million (who represent the greatest willful mass killing in modern times) would be almost an afterthought in their butcher's bill.
Ok point taken, my reasoning was that if they weren't exterminating jews they weren't exterminating anyone (yet), so if they loose the war they wouldn't be the cliché of genociders.
In the nazi victory scenario however they would obviously be. But IOTL they were trying to destroy all evidence of their killings. That's why they favored gassing-cremating over simply shooting people in the head, as it leaves no traces, just witnesses. Once the job is done, it's like in Fatherland, bury the secret....