She shouldn't have lived longer.I think there were interesting things they could've done with Kes's lifespan. She did meet the Ocampan colony whose people lived longer than the Caretaker's Ocampans, already setting up a situation where her lifespan could be expanded, and Kes seemed like a real prodigy, I think there's every chance she'd have found a way to extend her lifespan even longer.
It's what made the character alien and why she had to be so smart.
She had to live life in one day, her wisdom came from the fact she saw thought bullshit cause she didn't have the luxury to get caught up in pettiness. It's why she dumped Neelix. His weak over protective nature was beneath her. She was rolling her eyes at him in "Twisted".
Kes lasting all 7s and dying in the final ep. would have been a better and emotionally stirring finale than what we got in "Endgame". Imagine if Kes' good-bye in "The Gift" was actually in shows conclusion instead? There wouldn't have been a dry eye in the house and the Voyagers conclusion would have been raved about!