Leaving Korra without powers would have been as decisive an ending as any. It just wouldn't have been happy.
I don't think it would've been decisive, because it would've raised more questions than it answered -- like, will Korra ever access her spiritual side? Or if she's not the Avatar anymore, what happens to the Avatar cycle? Who will restore balance to Republic City, or the world, if there's no Avatar? Anything that leaves so much unresolved is, by definition, not decisive.