Kes was never boring? Then they wrote the character out because...?
Let's see. The Ocampa somehow "forgot" they had -massive- telepathic and psychokinetic powers, and instead of using those powers to further their race or at least repair the damage the caretaker accidentally did to their world, they became docile, flaccid and chose to live a subservient child-like existence underground. Makes tons of sense.
Kes is boring, childlike, and uninteresting. Partly because of the character, partly because of the way the actress played her. She's given a last-chance makeover (longer hair, etc) but she still isn't attractive enough or interesting enough to generate any viewer interest or good storylines at all, so the character is written out and replaced, thankfully with a far more interesting character that can actually add something interesting to the show.
The lifespan and the one child aspects of the Ocampa were the only interesting parts. Every life form can't be exactly the same as humans you know.
Kes was written out because those in charge wanted a "sexy babe" to attract male teenage viewers.
The original idea was to dump Kim but when a silly inside poll among the staff of a glossy magazine showed actor Garret Wang as one of the "50 most handsome", Berman's gang changed their plans and dumped Kes instead.
I don't find it impossible that the Ocampa forgot their powers. They were slowly brainwashed by their lazy leaders to become docile, flaccid chose to live a subservient child-like existence. This is actually happening in our world today too.
Kes is nice, friendly and beautiful but also strong-willed, determined, brave and curious whichmakes her interesting. She's a contrast but also a complement to more typical female action characters like Janeway, Torres and Seven. With her will to learn and explore, she personifies the spirit of Star Trek, a spirit which was gone when she was dumped.
Jennifer Lien did an excellent job in playing Kes. She really gave life to the character.
The "last-chance makover" you mentioned had nothing todo with any "last chance". The longer hair was a result of Jennifer Lien becoming allergic to the glue which was used to attach the Ocampa ears on her head and the new clothes was probably a result of Brannon Braga's obsession with women in catsuits.
As for the writing, the writers actually managed to come up with great Kes stories for three seasons and there were a lot of viewers who liked her.
The only interesting with the character which replaced Kes was her attributes.

And as I mentioned before, Kim was the one who they were going to dump.
The one-child thing was a stupid mistake. In a real scenario, that would lead to the species becoing fewer and fewer. Just count and see!
The lifespan was silly because a species with such a short lifespan could only develope into primates.