having an aasimar NPC does not guarantee that you will have playable aasimar. what makes you think of that?
1. In earlier version of EA there were racial tags for Aasimar so clearly a desire among devs for playable Aasimar
2. Aasimar aren't a monster race like Goblins, there are no Aasimar monster stats in 5e, it's a straight up PC race. They aren't going to build all those race mechanics and graphics for 2 NPCs only, they will use them for player characters.