This. If they've given your army some form of love, with a book, allegiance abilities, and mentions in the lore, then take a breather.
Furthermore, if we take the Heralds from Malign Portents at face value, then Moonclan, Nighthaunt, and Slaves to Darkness have all received new units in AoS, and why would they add models to factions they will get rid of? If you play Aelves, look at Daughters of Khaine. They could easily do something similar with Phoenix Temple or Order Draconis.
I feel that GW is making a common sense business decision, which is to honor the past while also providing a pathway for fans to still get models that aren't going to be produced anymore. They have to grow AoS, which inevitably will lead to older models falling by the wayside.