Most importantly, Tuvok and Neelix were dead by the time Tuvix came to life.
As I've said, I do not accept the validity of this statement at all. Consider that there is a very clear Trek-universe precedent for the situation in "Tuvix": namely, Trill joining. In that case, as with Tuvix, two personalities and two bodies are blended into a single composite entity. Would you claim that, for instance, Jadzia and the Dax symbiont both died when they were joined into Jadzia Dax?
I don't buy that comparison. A Trill enters the joining voluntarily, he's aware of both parts prior to the joining and the composite entity, so in a sense both Trill and host continue to exist. Tuvix didn't. His was a real composite personality. And he wasn't just a combination of those 2 characters, he became a character of his own, made his own experiences. And the decision to let him make his own experiences IMO precludes any right to deny him life afterwards.
Kes being the deciding factor... I got that impression as well. And honestly, while I really like Kes, that makes Janeway's decision even more random and selfish. Why not just help her deal with the situation - shouldn't someone with maternal instincts have done that? And it's a shame that we don't get to see the aftermath. I'd have loved to see Kes deal with what happened...