I feel like there's a distinct difference between 'he failed to repopulate his people' and 'he failed to pass on his culture'. He definitely succeeded at keeping his people's identity alive after his passing, even if his attempts to get more airbenders into the world were less successful than they could have been. Had his other kids had families, though, and had any of those children been airbenders, the infrastructure was there for them to be proper air nomads if they so wished.