Well, if we have to nit-pick, then all of the characters should have been dumped after season 3 because none of them had any arc to talk about.Why only Kes?
The Doc., B'Elanna & Tom's arc's continued thought out the series.
As for Kes.
Considering it was "popularity" that made Kim stay, maybe it she was the second character at the time on the low end of the popularity scale?
Maybe cause between her and Harry, they had the jobs that were easy to replace? Seven had Engineering skills and the EMH didn't really need a nurse.
She was starting to be treated more as a background character than a main one?
Me personally, I do agree with TPTB when they say they wrote the character into a corner. They gave her job that ended up limiting her participation and screen time. It didn't help that Picardo got the best lines which then allowed him to hog up much more screen time and dialog from her.
The fact that they made her afraid of her powers, instead of finding a better way for to use them limited her again.
I think her going from being over curious to being to wise too soon limited stories of her growing up.
The Kes we saw in "Twisted" IMO should have been how Kes was during the first few seasons. The wiser Kes was saw in s3, should have been s6 Kes, by that time she's learned and seen more. S3 should have been teen Kes, a little more rebellious and bit more of a risk taker.
However keep in mind, this is all in retrospect.
It's easy to fix mistakes looking back on them, much harder to see mistakes as there actually taking place in the moment.
I think in general, it's why it's so easy to criticize Voyager as a fan because we're all looking back on comment on how we'd fix it after the fact. I doubt that had any of us been writers on the show during production, that we would have caught the missteps as they happen.
In a way Voyager wrote itself into a corner where it couldn't develop any characters.
TNG and DS9 characters got most of their development externally. The farengi with Zek and Moogie, Kira with the prophets and various Bajorans, Data with Lore and Soong, Worf with other Klingons. In Voyager even having the Kazons as recurring characters was a little absurd, there's no way they should have been able to keep up with Voyager's speed over two thousand light years. They brought each character to the logical finishing point of their initial arcs in the second season. They had two choices: Bring in a new character with an inexhaustible arc, or break the premise to allow recurring characters from outside the ship.
Honestly until they introduced Cullah, wasn't it a different Sect. each time?
However, if the Kazon were more of a ruthless threat like the Vidiians, maybe we wouldn't have minded seeing them as much? Besides flying around in huge stolen ships, the Kazon weren't much of a threat. They were just kinda annoying.