Not necessarily. A culture that include the concept of a "raiser" - an octopus with the job of raising the babies, and passing the culture on to them, without mating at all - can avoid that issue. The "raiser" would also improve his average genetic fitness if he is a sibling of one of the parents, since the children would then all have approximately one-quarter of his genes.

(it's unclear why this evolved)

If it's not enough to kill off the species, evolution generally won't drop the feature.