It got better, yes. but not because Kes left. Kes suffered along with Chakotay, Kim and Paris as not being developed by the writing staff. Although personally, Kes was not an interesting character to me. How many psychic characters does a franchise need? They could have, however, done something along the lines of her gaining incredible power and having to adjust to being around those who didn't have the same abilities. She easily could have been distanced from the crew, like 7, and needed Janeway's help to reconnect to those around her. Of course she wasn't the eye-candy hottie that 7 was, so why do it. Besides, Borg are cool. The issue isn't did 7 replace Kes, it's that the stories got better after season 3. Not Kes's fault.