The interesting thing is that the loss of the ability to write for Kes was very sudden.
I see the many changes to her character as signs the writers were
scratching on how to use the character effectively.
Again, if someone can point out stuff said about her being hard to write for long before the fourth season, then I could buy that argument a little more.
Interviews around the time of "Warlord", IIRC. Breaking up Neelix and Kes was an attempt to liven/freshen things up a bit, since negative viewer reaction to the sexually immature Kes (until "Elogium") and "dirty old man" Neelix was causing concern.