As for the Morgenthau plan, well its basicly the Blood and Soil ideal the Germans were fighting for minus the slave labor.
Most Germans thought they were fighting to build an Empire. They did not think they were fighting for slavery in fact slavery was sold to them as a temporary wartime measure to make up for major manpower shortage and was quite unpopular still. They also did not see it as a genocidal war of colonization the way you see it as and the way Hitler and Generalplan Ost planned it as. In fact talking to a German who fought in the East it was sold to them as Stalin is going to double cross Germany so Germany must double cross him first.
It wasn't sold to them as we need that land to put German settlers on their land, kill off a large portion of the population and make what is left of the population surfs. There is a huge disconnect from the ideas Hitler had and what he sold his policies as being to his population and troops.
The problem with collective punishment politicians like Morgenthau and others is they could not understand that under Totalitarianism the population is rendered ignorant and all information is controlled. One can commit mass murder on an epic scale in Maoist China or Stalinist Russia or Nazi Germany and the population be unimaginably ignorant of what is going on.