What is most central for the character Kes is her personality, not the lifespan.
She is nice, friendly and beautiful but also strong.willed, determined, curious and brave. She has a way of getting her will and solving problems by simply moving and attacking the problem from another angle and can use violent methods only when it's neccessary to protect herself and her shipmates. In that wasy, she's both a contrast and a complement to more typical female action characters such as Torres, Janeway and Seven.
The lifespan was obstructive in a way. I mean, what if Voyager had become so popular that they have made several movies taking place after the homecoming. Not to mention the books and possible stories which are taking place after the ending of the series. Shouldn't Kes be in those? I found the lifespan obstructive and therefore I did away with it! Since there were hints in the series that her lifespan could be prolonged, I see nothing wrong in prolonging it. If Kes had stayed in the series, I'm sure that those in charge had come up with an idea to prolong the lifespan too.
As for ending up with a different character, no one complained when The Doctor got that holographic emitter. Now that was a major change of a character!