I would really be interested in a source that supports this point. I have a hard time reconciling this with German/Nazi tendency to document everything. Documentation is generally a function of bureaucracy as it is the primary reason for such documentation. Hitler might have been the lynchpin of the Nazi party but I think the German state would have been a little more resiliant. This is particularly since, as of this TL, it would not have suffered under strategic bombing, which destroyed so much infrastructure. But, I suspect you are more knowledgeable on the subject than I, so I am interested in your sources.
Ian Kershaw writes a lot about the structure of Nazi Germany, and I draw most of my conclusions from his works, along with various information about how its government functioned. Biographies on Stalin also offer insights into the important differences between the two systems of rule. Essentially the German government was divided into numerous competing factions, all of which vied for power and were led by a Plenipotentiary; by 1944 these were Speer, Bormann, Himmler, and Goebbels, but ITTL such centralization is unlikely without the pressures of war. These factions were filled with their own subfactions, ambitious subordinates, etc all looking to seize power. This constant competition and jockeying for control was kept in a delicate balance only by their mutual loyalty to Hitler and adherence to his leadership. Despite this, as time went on Nazi Germany became an increasingly confused mess with the opinions of Gauleiters, Party officials, factory managers, etc replacing bureaucratic rule by law. Remove Hitler from this picture and there's nothing to control the competition and chaos in the Nazi system; no one has the means or power to take and hold the position of Fuhrer, because they lack the system of control that Hitler had built.
This isn't to portray Hitler as a skilled manager or heavily involved in government; just the opposite, he mostly left things to his subordinates. But his personality and the general idea of what he wanted, along with the loyalty towards him (All of his inner circle owed everything they had to his personal support and favor), created a powerful system of control which IOTL was only broken by his suicide.
Nobody argues that hypothetical replacement German leaders could not try that in a victory scenario - without the Führer. The point is whether they would.
Note that by 1942 already - with war going on - the chances of surviving one of the worst possible situations, probably the worst after that of being a Jew, i.e. being a Soviet POW, greatly increased, not because the Germans had had a change of heart, but because they realized they needed HiWis. In that victory scenario, they would need serfs. Dismantling the conquered cities would also mean losing the manpower for their industries. In words of General Leykauf, certainly a great killer within the Ostplan, and yet a pragmatic man, if they killed millions, "who is actually supposed to produce economic values?"
At the end of 1941, German officers tasked with policing the rear areas voiced grave concerns about the chances of maintaining order, when the populace was starving.
An all-out Hungerplan, leaving aside all ethics, did not come without serious very practical drawbacks.
I don't expect the German decision makers to suddenly grow a heart. But I wouldn't be surprised if they saw those.
The reason for the declined rate of death among foreign laborers and POWs was due to the worsening situation of the war in the east. As the war had gone on much longer than expected there was a massive shortage of labor which needed to be filled. Thus at the same time as mass starvation of laborers was temporarily halted, large number of new ones were brought in, reducing total percentages of deaths. However, at the same time as this rational use of labor, genocidal practices continued unabated. The very concept of "performance feeding" (Denying food to under-performing workers, resulting in their deaths) was designed to continue killing off those too work to weak, resembling the program of separating those unable to work to be sent to the gas chambers immediately while those able to were kept as slave labor for a while longer.
With the war ended there will no longer be a pressing demand for labor (Especially as armaments production slows down and millions of soldiers return to the workforce) and mass slaughter will resume. Slavs will continue to be employed as slave labor; Generalplan Ost specifically set aside percentages (35% on Ukraine, 25-35% in Russia, etc) to be kept alive. However, they will be performing tasks of a different sort than workers did in Germany. While killing a worker in a factory would not be beneficial as it would take time for a new worker to become acclimated to the job, a worker building roads and digging ditches could be replaced easily. The tearing down and building of infrastructure in the East was a central component of the Generalplan. With labor readily available no need to keep workers alive, millions will be worked to death without slowing the projects. Those that survive will either be shot or kept as slaves for SS-run factories or plantations, perhaps eventually Germanized.
The major concerns in implementing the Hunger Plan were always related to not having the resources to do so effectively. Sealing off a city and destroying the food market is next to impossible without serious force, especially when the population has no reason to accept its death. With the Heer no longer involved in fighting elsewhere this problem will be resolved. As Nazi treatment of the General Government shows, delays and setbacks did not result in a halt to the killing; instead they simply looked for more efficient means to do so.
The economic concerns about the mass killing of tens of millions, which a rational mind would of course note, played very little part in Nazi policy or the Heer's cooperation with it. As I noted before, the goal was a restructuring of the East to benefit Germany with a food surplus and open land for settlement. Generalplan Ost involved not only agricultural but industrial colonization as well; while farmland played an extensive role, new "Germanized" cities were to be industrial centers as well. Thus any economic damage in the short term was, theoretically, to be replaced by an immediate benefit in food for Germany and economic growth in the long run as the East was settled.