I’m not clear what motivated Sauron beyond a desire for domination. No, I don’t think he was actively genocidal, but I think he would have been quite content with a less than ideally productive slave economy wherein his subjects were miserable. His human allies, the leaders of the Haradrim and/or the Easterlings would have more concerned with return on investment. OK, they wouldn’t think of it in those MBA terms, but ultimately, having sunk a lot of their own blood and treasure into their war in the west, they would have wanted something beyond the misery of the conquered as their reward. Within a few generations, left to their own devices, they might well assimilate into the cultures of the West much as the Normans did with the Saxons running England at the time of the Conquest, ending with some hybrid cultures and some very interesting linguistic features. Alternatively, the Gondorians and Rohirrim might be driven into the marginal living areas, as the Celts did when the Saxons came to Britain.
Sauron didn’t want to slaughter the population of the West, but I don’t think profit was his driving motive. He wanted Dominion, and he wanted Fear. I don’t think the bottom line was particularly important to him. So, while his generals and viceroys might be open to negotiate with the conquered peoples, Sauron himself would not. But that probably wouldn’t stop his generals and viceroys from doing it anyway. Underlings have a tendency to find ways to line their own pockets, and historically, it seems that the more dictatorial the society, the greater the corruption. As Grey points out, Sauron might well want to eliminate the scattered remnants of Men who were connected culturally to the Elves or to Numenor (the upper class Gondorians, and the Dunadain), but the Rohirrim were not. They didn’t know from Eru or the Valar; their ceremonial observances and afterlife concepts were all about their ancestors. And, again, the Men allied with Sauron would not have cared about such things beyond that required of them by Sauron (Black Numenorians excepted).
All of which is to come back around to my point that the Rohirrim might do better as a cohesive people with a recognized leader than without in a Sauron-dominated Middle-Earth, that most of the people of Rohan would certainly feel that way, and thus that Eowyn was in fact ignoring her duty when she rode to the Fields of Pelennor. This was in response to MrSmith’s question as to what point there would be in her staying behind to lead Rohan as she had been told to do by Theoden.