Futaba and Kaoruko always struck me as more codependent than purely toxic, in that they enable each other's worst tendencies (passivity and arrogance), but only around each other, because every other person in their lives has saddled them with impossible expectations and so they need some kind of validating outlet.
KareHika are childhood sweethearts, MayaKuro are rivals with benefits, JunNana are a newlywed couple with some dark secrets, but FutaKao is a ship that's in the late marital stage, simultaneously deeper and shallower than all the others. So it's only fitting how the resolution to their troubles is not sappy self-sacrifice, but a remembrance of what made them so twisted in the first place, and an agreement to not change something that's kept them happy and sane for so long.
Without Futaba, Kaoruko would've ended up like Maya, and without Kaoruko, Futaba would've had no purpose in life and no role in the show. Togetherness isn't the best ending- it's just a different kind of conflict for a different kind of plot, all within the greater scope of the disaster-lesbian-epic that is Starlight.
Rather long post, but this ship barely gets any discussion beyond the odd 'toxic' meme, so I wanted to share my perspective on it.