On the one hand adaptations for sperm competition, such as large testes, mate guarding, etc. are used to support that female in species are promiscuous.
On the other hand we know multiple eusocial species where most inidividuals don't reproduce letting their kin (e.g. the bee queen) reproduce. Which means that in some cases it is more beneficial to let your kin reproduce than to do it yourself.
For that matter, would not there be intermediate states, where multiple indivuduals reproduce, females are promiscuous, but since reproducing as much as possible yourself is not necessarily good, sperm competition adaptations would not get developed? Are there any candidates for such species?