That I can understand. However, the OP in that thread you linked reported:
But, in this thread, @Apollo only asked if the Achievement would disappear. They did not say anything about missing the Ethereal Transmogs.
(Perhaps players should be more clear what they’re asking or reporting. )
Edit: I hope anyone who lost the Ethereal Transmogs when the Achievement disappeared can earn them (the Transmogs) in Season 32. (Even if the Achievement disappears).