It is inevitable that more people will be able to exert more control over their reproduction. I agree that this is likely to amplify the human tribalism that already causes so many problems. I still don't see how that inevitably leads to “20th century eugenics” nor why we should discourage or fear research.

You haven't had children, have you? They don't neatly fit into plans or develop to top down desires.Thinking the children will be superior is extreme arrogance even if it all works out and they are born immune to cancer. It puts too much faith in one advantage determining all.

Under that catastrophic social model the elite should have been descendants of the elite from before the Bronze Age collapse, and heavy cavalry should have maintained their combat effectiveness over the masses even during the Infantry Revolution.