I'm not an evolutionary biologist, but this article (https://en.wikipedia.org/wiki/Evolutionary_game_theory) mentions Hamilton's rule: A given altristic gene will propogate if "the genetic relatedness of the altruistic donor exceeds the cost-benefit ratio of the altruistic act itself". Is this not widely agreed upon?
But your point still stands, and has clear parallels in other cases, like sickle-cell anemia: one copy of the gene is helpful, so it remains in the population, but two copies is harmful, so its spread is limited.