The arrival of Seven of Nine on Star Trek: Voyager marked a major shift in the series. It brought a new era to the show, while simultaneously bringing in one of the most iconic characters in both design and execution. Yet, to bring in such a major player, meant someone had to be shown the door.
Enter Jennifer Lien's Kes. Kes was an Ocampuan with the fatal character flaw of only having a seven-year life spawn. This meant that the character would die long before the ship ever reached Earth, or assuming the show still went seven seasons, would die about the time the ship landed. There was some nonsense in the middle of her run on the show that "extended" her life a bit, but even then, she still had fatal flaws.
Firstly, fans couldn't get past the one-year-old Kes dating a 30-something Talaxian named Neelix. This relationship dynamic caused there to be a lot of unnecessary drama, specifically with how jealous Neelix was. The coupling held the show back and fans were turned off of them as a whole from the moment they were introduced.
Then the character became so soft and gentle that whatever unique and different personality she had felt jarring and unrealistic. For most of the early show, she followed Neelix around like a lost puppy, and even when they tried to bring in outside interests like being a nurse or helping grow food it still felt forced. Her interests seemed like a desperate attempt from the writers to distance her from the soul-crushing relationship that she was shoe-horned into.
She could've been an incredible character had she not been forced to deal with the awful Neelix relationship, but the idea to pair them was so bad that there was no going back. It was so bad, it took five or so seasons to fix Neelix. He didn't come alive until he became a custodian for the kids on the ship, namely Naomi Wildman.
That was something that Kes just didn't last long enough to get and enjoy. Hence why Seven of Nine's arrival was so impressive. She didn't come in with the same limitations or poor attachments that doomed Kes and Neelix.
She was a bit of an enigma, with an unpredictability about her that made her exciting in ways Kes never was. That harder edge, especially introduced so late into the show, freshened up the entire roster. She added such a new and unique element to the show that every established character got freshened up by proxy.
So when deciding what character had to do to make room for Seven of Nine, obviously Kes was the best choice. She was so marred by bad writing that it would've taken a Herculean effort, dare I say a miracle, to fix her. The swap helped the show's image and made things all the better.