I actually think this move was pretty savvy on Korra's part. Season 1 showed us a world where Science and Technology was quickly turning Benders into obsolete anachronisms… We saw how the non-benders all too easily signed on with Amon's anti-bender pogrom.

By opening the door with the Spirit World, Korra is reintroducing Magic and Mythology into the Human World, "restoring balance" to a society that had leaned too far towards cold scientific rationalism. And, perhaps not unintentionally, she guaranteed that society would need Benders for centuries to come, in order to keep the peace between Human and Spirit.

PS, I was originally shocked when Korra was cut off from the Old Avatars, but I have come to think of this as a good idea. Each decamillenium needs the Avatar cycle to start afresh: Korra is Omega and Alpha, last of the Old Avatars, and first of the New Avatars.

Question: at what point did they forget about Wan? Because the whole" world-ending disaster that only you have the power to prevent" thing is exactly the sort of information that needs to be passed on.