I have to disagree here. Kes is a smart person who has the ability to creep under the skin on the person she's trying to change. She will also attack the obstacle from another angle. Watch her in "Jetrel" when Neelix is wallowing in grief and self-loathing about not being a war hero but a deserter from the army and you'll see what I mean. There are other examples too, like when she's discussing with the "bad Doctor" in "Darkling". OK, she wasn't that succesful then due to her trying to deal with an evil entity. But she would probably be more successful with Seven. Of course, Seven would be obstinate to start with, but in the long run Kes would win her over and they would become friends. OK, maybe the best buddies but they would definitely have respect for each other.
I actually think keeping Kes on the ship with Seven would have complicated things for the writers. They clearly wanted to build a strong Seven-Janeway relationship and have Janeway as the principal figure helping Seven regain her humanity, with The Doctor as a secondary. Showing Kes helping Seven in any meaningful capacity would have muddied the waters a bit.
Skilled writers wouldn't have any problems with that. But since the Voyager writers were half-wits who could only type the word "Seven", then I understand that it would have been problems.
Given that they couldn't even figure out that an ensign is supposed to make lieutenant after a few years, I find it difficult to disagree.
Maybe by the end of the show, but seven didn’t have many relationships to speak of besides Janeway and the doctor for the most part. There’s also the fact that Kes’ lifepsan is short. Seven might have thought that it would be wasted to develop a friendship with someone who would die so soon.
It would definitely have taken time to develop. I imagine Seven would've resisted getting close to Kes as she did everyone. I could see her having blunt conversations with Kes about her short life which would have Kes explaining how that just made her more determined to make the most of her time, and Seven would see that Kes was a valuable part of Voyager's crew. I can imagine Seven despite intellectually knowing it was coming being emotionally blindsided by Kes's death when it's coming - and in the end Kes would've ascended to a higher being at the end. Voyager really missed out on the pay off of Kes's series arc when they let her go early in exchange for keeping useless Harry.