The Nathrazim originally weren't necessarily demons though, as we even see an uncorrupted one in Legion, and the void lords can't have created/converted them since demons are corrupted by fel and not by shadow which is what the void lords use. Nor is there any examples of a servant of the old gods/void being able to turn against them IIRC so even if they did manage to create demons that doesn't explain how Sargares was able to turn them, and other demons if he didn't create them, over to his side to join the Legion and fight the void lords. The only examples of prominent demons we have (apart from the pit lords who aren't shown to have any role in corrupting Sargares like the Nathrazim had and have virtually no backstory) are ones who originally weren't demons like the Nathrazim and Eredar.

This would work if there's another fel magic wielder out there that is powerful enough to create demons that existed before Sargares but there's no hint of that existing either in Warcraft lore AFAIK