My theory:
The Planet is all about balance. As long as there is a bender of each element, there is balance. If one "part" is removed, then it becomes unbalanced and the Spirits have to restore the balance. They will do this by "granting" the power of airbending to a new bunch of nomads, who will become the new Air Nomads. Then balance will be restored.
Why this hasn't happened already: because there is still one airbender left, even if he is frozen in an iceberg. As long as there is one, then there is balance.
So when Aang dies, and gets born into the Water Tribe, a new group will become the Air Nomads and will??have plently of time of master airbending by the time Aang comes to them to learn.
