I seem to recall that Kim was originally supposed to die in "Scorpion," and they had a choice between Garret Wang or Jennifer Lien leaving the cast to make room for Seven. So Kes was written out instead, not very well in either "The Gift" or "Fury."
I disagree, I think "The Gift" & "Fury" were good send offs for the character. It would have been much easier to simply kill her off much like Yar & Jadzia Dax but they didn't take the easy road. They allowed a character that was falling into the background to have an epic exit by becoming a powerful being and still remain alive, giving the writers and Kes a chance to return in another series, film or to continue in stories.