Airbenders theory

Honestly, while I do believe that the vast majority of the airbenders were killed in the air nomad genocide, I doubt all of them were and that there weren't any survivors at all. While it's true that the fire nation laid traps for them and hunted down every single airbender they could find, motivated by the fear that the avatar might have survived, I think that's even more reason to believe the airbenders had to hide their identity even more to stay alive as those who didn't, died.

Therefore as they abandoned their culture and practices and thereby their spirituality I actually think their airbending weakened severely just like Kyoshi's mother, who also had abandoned the lifestyle of air nomads and monks and needed fans to be able to airbend. And we know that airbending is strongly connected to spirituality, because it is precisely this that causes every air nomad citizen to be born an airbender. So it might not be too far-fetched to think that the weakened airbenders died after 100 years and had non-bending children with the local populations of the places they fled to, who couldn't airbend because of their and their parents' loss of spirituality.

Therefore, Aang should still technically be the last airbender, but I think not all lineages of the air nomads were eradicated and killed and there were some survivors. This however does still mean that their culture and way of living were completely wiped out, which is still devastating and cruel, not to mention all the lives that were lost.

I think therefore that when Korra opened the spirit portals, whereby a lot of spiritual energy and spirits started flowing into the physical realm, this change in spiritual energy in fact could have awakened the airbending potential in the descendants of these airbending survivors and any other non-bender who at some point even further back than the air nomad genocide had had airbending ancestors. After all, we know that bending is heavily tied to genetics, but also doesn't come out of thin air either, but has to be bestowed by the lion turtles, otherwise there would have been human airbenders or other benders during Wan's era since they probably would have been even more in contact with spirits.