Jeri Ryan was a huge addition to the cast of Star Trek: Voyager. In fact, she was arguably the biggest late addition to any television cast ever. Maybe only that of Jonathan Frid in the daytime soap Dark Shadows can rival the impact Ryan had for a show well into its run. Her arrival was a boom period for the show, with plenty of hype and goodwill going to a show that needed it.
That said, Ryan's arrival as Seven of Nine wasn't what improved the show. It was the fact that Ryan was coming into a series and replacing one of the major names that the show couldn't seem to do anything with; Kes. Played by Jennifer Lien, Kes was the worst character on the show and maybe the worst character in Star Trek history.
Her look as Kes was awful, featuring a pixie cut that made her feel like a Wish.com version of Tinkerbelle. She was just two years old when we met her, yet old enough to have an adult romance and travel the stars. That's because her people, the Ocampa, age fast. So despite being just two, she's lived 20-years of her life.
That whole fact made it creepy when we found out she was dating the nearly 40-year-old Neelix. A man who would be nearly twice her age if she were 20, now he's 20 times her age. It was unsettling. It didn't help that her character was given a time window of living no longer than the show would go. After all, if her series only lasted seven years, then she'd likely die around the time of the show's final episode.
Seems unnecessarily dark and tragic. That's still not the biggest issue with the character. She was often quiet, unassuming, and boring, only showing any personality when her magic powers started to act up and influence her negatively. She was a badly written and conceived character, played by an actress who didn't have the impact needed to bring new fans in.
To be blunt, Kes held the show back. She as a character was bad enough but she didn't even raise the standards of those around her. Other characters like The Doctor, Neelix, and even Kathryne Janeway were so much better interacting with Seven of Nine as opposed to Kes, only because of how their collective personalities worked in contrast to hers.
Kes just didn't have the depth or intrigue needed to carry storylines on her own. Honestly, anyone replacing Kes was likely going to be an upgrade over the former cast member. Replacing her with someone who looks and acts like Ryan does is just a bonus.