Well yes, almost every delinquent has a sad backstory, but that Hisoka is going to finish them off seems fine to me, and speaking of Hisoka he also has a sad backstory but his character is unique. As for Gon and Killua yes, they were more developed than Kurapika and Leorio, but I think Togashi planned to do more with them, however we may not see it.
I understand your feeling of emptiness, and well that alternate ending to me left me more empty haha.
Wait–what? Hisoka has a backstory? Gosh, I need to read it!
That's right, it explains how he learned and developed his nen and everything else, especially his obsession with chewing gum, hence its importance.