I guess one of the possible reasons Futei and Kaine lives is to set another possible course of conflict - either for Karyo Ten or Kyou Kai - in the future. That's it, really. This universe keeps expanding, but it's only right, especially with Hara planning 100 volumes for Kingdom. I was a bit furious that those two would live, but it's not like Futei and Kaine are weak in the first place. Just like the Hi Shin unit, they're veterans of their own merit.