I think it's an interesting idea. I could see Kes helping rehumanize Seven instead of the Doc, perhaps using her psychic abilities to benefit somehow. Obviously, Seven would probably become the Ops officer, which frankly makes more sense than giving her a non-position as they did.
I wonder if they would have actually let Kes age? If so it would have been interesting if she transitioned from a sisterly relationship to Seven, into more of a mentorship, and then finally Kes dies and the first real human moment Seven has is this - she cries. It would also be interesting if Kes turned into a Jean Grey and came back to life via her psychic powers.
That would have been cool.
I do agree with some of your points here. If someone would have been a great support for Seven, the it would have been Kes.
Just remember The Doctor in the beginning of the series. He was nothing but a hologram, programmed with bad personal behavior and almost everyone on the ship found him annoying. But not Kes!
She showed compassion for The Doctor, she told him that he was actually a sentient being despite being only a hologram and helped The Doctor to start looking at himself as such as well. She also made the crew start treating The Doctor with respect and in a short time, everyone saw The Doctor as a colleague instead of just a program. Not to mention that she actually saved The Doctor too, in "The Swarm" when she came up with a brilliant solution for the problem.
If Kes could do that for a program, she would have had even better chance with a living person.
As for the aging and death scene, I could live without that. I've had more than enought of such scenes in syrupy soap operas. Besides that, as I've pointed out before, there would have been problems with the make up. Actors with make up which would make them look old just look terrible, it looks more like some disease than real aging.
As for the "Jean Grey" thing, now that's actually interesting. But a prolonging of the lifespan would have been better.