This has been discussed before but Kes was never originally going to be written out of the show when they brought in 7 of 9 on. In fact there were already season 4 stories in the works for Kes. It was to be Harry Kim who was going to be killed off. Some of the producers didn't like Garrett Wang because he was often late for work and didn't bother to learn his lines (he was actually suspended from the show for two episodes in the third season for bad behavior) . But during the summer break between seasons 3 and 4, Wang was voted as one of People Magazine's 50 most beautiful people, so UPN basically told the Voyager producers they had to keep Wang. The decision lay with Jeri Taylor and she decided to write Kes out because she didn't like the character.
Jennifer's departure from the show was very messy. The whole cast knew one of them was going to go and once it was learned Garrett Wang had been saved, everyone thought Ethan Phillips would be let go. So much so, that Bob Picardo and Tim Russ were actually planning a goodbye party for him. Ethan Phillips and Kate Mulgrew talked about this at a convention I went to back in 2010.
Jennifer had no idea she was being written out. She went to work as normal the day production was to begin on The Gift and on her script there was a post-it note from Rick Berman that said something like "We'll miss you."
While Kes was a secondary character, I do felt the show lost something when she left. It would have been great seeing Kes and 7 interact but it wasn't meant to be.