The short life span was a chance to do something different with an alien species, but they seriously dropped the ball with Kes, a lot.
The weird pregnancy thing doesn't bother me, but the single pregnancy only for Ocampas would pretty quickly dwindle their numbers down, since every pair only reproduces one baby to replace them, and not every child will reproduce due to early deaths and life choices. I pretend that's something the Caretake did to them to reduce their population with the intention of fixing it later when he got their numbers down to whatever he wanted. I love Kes's mental powers, even if they were poorly defined, a lot of interesting things could be done with them. I am a sucker for that kind of sensitive mysticism though.
If they'd kept Kes around I like to think some interesting things could've been done with her. She could've gone on to have a baby like she did in The Year From Hell (but please don't let the baby marry Harry, that was sooo creepy since she'd have grown up with him like an uncle), and by the finale she could have died of old age after making some valiant sacrifice to save everyone/get them home, or she could've evolved past the limits of her species somehow.
While I hated her relationship with Neelix, I did kind of like that Kes could love him, it just showed how emotionally evolved she was to see past, well, everything, and was so capable of loving. I do wonder how things would've gone if she had been the one to wind up with Tom instead of Blanna.