But instead they didn't know what they wanted to do and were retarded about it. It would have made the most sense if people could theoretically learn any kind of bending and the bending being more specific like sandbending or swampbending was cultural/knowledge-based. Then the destruction of airbending would have been the result of the burning of their libraries and massacre of their teachers/monks instead of "all these dudes who have some kind of spiritual-genetic capacity for slapping people with their farts are dead". Aang already had the airbender state so it wouldn't matter if someone could theoretically learn the other forms of bending, but I assume it would be explained as "it takes a lifetime to master one form and would be pointless/impossibly difficult to halfass the other forms anyway" and the avatar's advantage would be the spiritual memories of multiple lifetimes of bending.
But in any case Bumi and his sister were fucking useless. And Tenzin was married for like, what, 14 years since Jinora was like 13 or so? He could easily turn his wife into a human termite queen with a dozen kids. It wouldn't even be unrealistic for the era, the writers just didn't have the balls to be like "haha woman can have a dozen kids lol".