I think Paris became boring in the later seasons. Before that, he was a great character.
As for Kes, there was never any arc or any "Dark Phoenix" scenario. The two occasions where she "turned bad" during seasons 1-3 were "Cold Fire" and "Warlord" and they were both isolated events, due to outer forces. In "Cold Fire" she was tempted by the evil Tanis but in the long run she stood up against him and saved the ship from destruction. In "Warlord" she was controlled by the warlord Tieran who was in possession of her body and mind but she stood up against him too and was able to defeat him with a little help from her friends. After both those occasions she became her old self again.
Jean Grey was being manipulated by Mastermind and the White Queen, thus molding the the Phoenix Force they believed to be in her.
Many of Voyager's stories are based on or are similar to other works of sci-fi.
...but if Kes has no story arc., then I guess that could be another a good reason to replace the character too.
Voyager had enough stagnant characters like Chakotay, Kim, Neelix and Tuvok, who also had no story arc's of their own. Interesting how 3 of the 4 don't get high marks among the fan base possibly due to it.