Besides, I have believe that the Avatar spirit knows what it's doing. Even if Aang was unable to spawn some new Airbenders, I figure that down the road, once Korra dies and her Earth-bending predecessor dies and then his/her Fire-bending predecessor dies, the Avatar spirit would simply be reborn into a brand new Airbender.
Being the Avatar ties you to all of the elements. Korra is the Avatar; I imagine her offspring could potentially be Benders of any kind, even if she was originally from the Water tribe. How many Avatars do we know of that actually had kids?