I can't help but wonder how the show would have turned out if Harry Kim was written off and Kes remained.
Indeed, Garrett Wang was supposed to be the one fired, not Jennifer Lien. The execs changed their minds when he was named one of People's 50 Hottest Actors in 1997.
Year of Hell was supposed to be the season 3 cliffhanger, leading into an arc in season 4. This is why there was a fair amount of Kes development leading up to the season finale. She was supposed to take on a larger role. Indeed, she was supposed to be the one guiding and nurturing Seven's humanity along with Janeway, as well as continue as the Doctor's assistant (which Tom Paris wound up doing).
Garrett Wang was disliked by the execs because he was considered lazy. Late for meetings, rehearsals, filming. Plus, he was a weak actor.
Everyone's contract - except Mulgrew's - was done at the end of season 3.
They had already signed Jeri Ryan, and the execs didn't want the cast to grow to 10 members, so someone had to go..., what didn't stop producers to hire 4 young actors from s6, who served to nothing, except the one who played Icheb, who was kept until the end of the series.
There's some mystery behind why Lien herself was fired. Rumor has it she developed an allergy to the glue used for her ears, which is why she changed hair styles. There are some other rumors, as well, such as drug use on her part and her rebuking the advances of a guest star somehow related to one of the execs. No one knows for sure.