The ending worked thematically - the story started from Jack's POV, with him regaining consciousness after the crash, so it should end with his death.

I also don't think, realistically, that Kate's life would have been that happy afterwards. She'd broken the conditions of her parole by leaving the country, and it'd be revealed that she'd lied about being Aaron's biological mother and about the other survivors. Her afterlife reunion with Jack suggests that she wouldn't move on with any long term romantic relationship, or otherwise she'd be reuniting with that partner not someone she'd known for about 3 years (every pairing in Lost has this issue).