You can't "lock" something that doesn't exist.

[Arrius]

We got a nice thing. We had to lose another nice thing to get it. I'd say have patience, and that the extra genders will come eventually.

They could've just kept one nice thing as a whole and announce that they got the other nice thing in the back, with a possible plan they get that one done by the next expansion.

SE at least attempted to implement the Au Ra into the world with the Doma refugee story here. And it also make sense for the Viera to be around, since the presence of Fran confirmed their existence.
The Hrothgar, as for now, haven't even been witnessed in the current live game in form of a npc or a single mention through dialogue.

Instead, they get the brute force 'they have been here all along' handwave/retcon when the expansion comes out. And that did cost the players the gender diversity they hoped and expected from the game.

That is quite a blunder they pulled off here, given how much they usually invest themselves in working out the lore.