I have little faith in Blizzard to do really anything right, but I don't buy that this is the actual end to Ny'alotha. The last phase of the Mythic N'zoth encounter suggests that the Forge of Origination doesn't actually work on him.
So for now, my assumption is that the end of the normal/heroic fight is a hallucination created by N'Zoth, and that Mythic is the true ending.
Even thematically, this ending doesn't fit with the ending of literally every other expansion ending cutscene. Every single one of our victories come at some sort of a price or twist.
WoTLK: There must always be a Lich King, so Bolvar has to take on the role of the Jailer of the Damned.
Cata: We defeat Deathwing, but the Aspects have to sacrifice their power and become moral.
MoP: We stop Garrosh, but the Pandaren force us not to kill him, which eventually leads to the Iron Horde being formed.
WoD: We kill Archimonde, but Gul'dan is flung into our reality and summons the Legion.
Legion: We beat Argus, but Sargeras stabs Azeroth, releasing Azerite and causes a new war to start between the Horde and Alliance.
This cutscene doesn't conclude the story of the expansion. It doesn't even have any dialogue; it's not even a full minute long. Blizzard's story telling has been really bad in this expansion, but I don't believe that it's actually this bad. I'm waiting until the Mythic encounter is downed before making my judgement.