You would be right if there was always status quo. Real life is not that pretty. Every 50 years or so there were great disasters, failed crops, starvation. Guess who dies first from hunger master, or slaves and their family?
Surely submission is vital in surviving too, but victory increases survival of your tribe and allows you to feed more kids than submissive people.
One can also claim that nothing really vanishes in nature. Even though Neanderthals were wiped out, some old haplogroups, Huns in europe, Illyrians, Prussians, Veneti, etc, there are some of their gens living still in us. So maybe it's all statistical after all. Few gens are left but their language, culture, believes, distinctive look, etc gone for ever.
A slave owner rarely had hunger...Lots of slaves lived better than many roman citizens. Humans are useful for humans, we don't (use to) kill the golden eggs hen without a serious reason. You take care of your production instruments -servi aka instrumenta vocalis-.
All those groups you have quoted were integrated in other societies. Their gens survived, their culture -or part of it- didn't to. I'm a spaniard and you are possibly a canadian of french origin, we don't speak gaulish, iberian, aquitanian or celtiberian, we speak romance. The romans had a hard work conquering our lands, but finally the survivors -masters and slaves- were latinized. The succesful warriors -romans in this case- settled here, but I bet you don't think you descend mainly from them even despite the high number of casualties among the gaulish warrior class.