Re: Fruits Basket's Ending...
Lots of people like happy endings. Heck, you should see the people who complain about the Fatal Frame/Project Zero/Rei games. The Rei (or Zero) in the title refers to the fact that the canon endings are "zero" endings, neither happy nor sad. Yet people who play those games still want a happy conclusive ending and whine about why there isn't one WHEN THE TITLE TELLS YOU THERE ISN'T ONE.
I actually despise the usual 'happy' endings because it's usually unrealistic and too 'clean.' I prefer more realism (yes, I know I'm saying this about a medium that spawns emo teens who can kill anyone they want with their cardboard rainbow robots).
Also, I'm not sure, but if I remember right, Furuba wasn't close to being done at the anime's end. If they made up an ending, how much annoying whining do you think there would be, regardless of the quality?
And lastly, I found the Air ending better than most animes. I kinda liked how they didn't force the issue with a happy ending. I actually found the Summer and Air arcs much more enjoyable than the coma-inducing Dream arc. Though in the end, the only Key story I truly enjoyed was Planetarian.