Well, he had the population of the rest of Eastern Europe to utilize. THe Reich was never going to permanently settle just for what it had managed to conquer so far. Hitler believed that a state either continues to aggressively expand or it get soft and the next stronger player wipes it out.
I have to agree! And thanks Calbear for the answer.
One has to wonder how long it would have taken and how many more lives would have been sacrificed for Hitler to have his "super city."