Humanity doesn't have to die after The Harvest.

[AnthroHeart]

From reading the material, humanity that doesn't get harvested will move to another 3D planet. I wonder if they'll have to wait another 25,000 years, or if they go based on seniority of their vibration to planets that might be harvested sooner. Perhaps all they needed was one more incarnation. I don't think that planets are spaced 25,000 years apart in terms of harvestability.