If you haven't watched / refuse to know the end of the Aether story, don't read any further.
Now I think we can all agree that the ending was not 100% what we were hoping for. We were teased about winning the Great War, defeating Dr. Monty, all that jazz. Hell, even Alpha Omega's ending told us we were in for a big fight.
But we got blue balled hella hard and left with what I'd call the most depressing moment in Zombies history.
Despite all the loose ends still remaining...I actually felt at ease with what we've been given as a conclusion. What people aren't understanding is why things unfolded the way they did. And to understand that, you have to look back at the end of Black Ops 3 - the Original Plan.
The Original Plan
Monty was able to remove the Apothicons, Shadowman, and undead into the Dark Aether. While he couldn't repair the multiverse, he managed to lock them up so there weren't any holes. A duct tape solution, if you will. He assumed that by closing off each reality, anything that existed currently outside of said realities would return to their origin point and no longer exist in his perfect universe. The children (aka the souls of the pure O4) would ensure that he could keep tabs on them at any given moment in any universe to prevent another disaster.
Except the crew survived thanks to the blood.
Monty was going to wipe them out (which would be the easiest solution) but instead sent them to the Great War. They would win, then die, and then be reborn later in the future during the 1900s. This created the cycle we know as of now. Monty purposefully kept them in the loop and knew as long as they stayed there, no more harm could further be done. It wasn't the ultimate fix, but he wasn't willing to potentially sacrifice himself to fully restore the multiverse. Again, duct tape solution.
But Monty knew what the only true solution was. He just didn't want to go through with it.
The True Plan
Monty AND Richtofen knew the true solution. But Richtofen also did not want to sacrifice himself nor his crew, so he tried finding ways to cheat death. After realizing it would only continue the cycle, he changed course and intercepted himself in at Alcatraz when he would've normally gotten the blood.
Instead of allowing himself to continue, he killed his cycle self and instead allowed Nikolai, the only one who Monty would be unable to see/track, to do what Richtofen was (and knew he was) incapable of doing: the sacrifice.
Since Nikolai opened up to what the Kronorium told him, he knew that none of them were to survive. The only way to restore the multiverse was to remove everything, including themselves. So he convinced everyone - Primis and Ultimis - to follow him because he knew the way to salvation. And he wasn't lying; he just didn't tell them what the end game would be.
This is why he wanted his friends' last moments to be with one another, drinking in what they thought was a pre-celebration to victory, thinking of what they wanted most in life. Nikolai wanted their sacrifice to end on the best note possible because none of them would make it out alive. Although I personally think poison isn't quite a happy ending (as seen in the final moments)...Nikolai did what he thought would be the quickest way to make this happen. There was no way to convince both crews to kill themselves on their own terms. As long as they existed, it didn't matter if they banished everything evil to the Dark Aether; they had to be removed as well.
It was unfortunate that even Victis had to suffer, as they were the victim in the grand scheme for what is considered "the greater good". But it was necessary.
EDIT: There are some key points that I missed when writing this post that I wanna add on.
A user on r/CODzombies brought up a very valid point regarding the cycle being broken and what it finally means.
We know that the cycle is what keeps Primis reliving their adventures, from death after the Great War to reincarnation when they all first meet. Many of us thought that drinking the blood (Richtofen's plan) was the key to breaking said cycle, but after BO3, it wasn't.
We were right to compare it to the MOTD cycle. In fact, BOTD also replicated this cycle in BO4 with Primis to show us AGAIN how to break the cycle.
In MOTD, the cycle continues if the crew kills Al - something that already occurred in the real world. But when Al kills his crew, the cycle is broken and they're released from their own purgatory.
In BOTD, the cycle continues as long as Primis remains. Even when they die, they're brought back by the Warden to relive their personal hell. When Richtofen dies, their cycle in purgatory is broken; HOWEVER, it required some past events leading to it (Primis Richtofen teleporting to BOTD from the Great War, Pablo giving him the Staff of Fire crystal) to start that breaking point. Catch my drift? BOTD Richtofen was perpetuating that cycle and needed to be sacrificed to escape it. That's how the cycle broke.
Now in terms of the big cycle, it's no different. Nikolai ended the cycle by sacrificing himself and the crews. The key to breaking these cycles was to eliminate the things that would continue the cycle. The mobsters killing Al caused the cycle, and had they to die. Richtofen kept cheating death to avoid direct sacrifice which caused the cycle, and he had to die. Ultimis and Primis were responsible for fracturing the multiverse causing the great cycle, and they all had to die.
Hopefully I didn't lose ya.
To me, this was ultimate kick in the heart. Because we knew our crews had to die, but we just weren't prepared for it to be like this. And personally, I'm slowly accepting that this ending is what we needed.
A straight one-shot kill to our beloved story, with the sad realization that the characters we grew alongside over 10 years needed to win the war within themselves...just as Nikolai said. They caused this madness, the fracturing of the multiverse, and they needed to satisfy the natural order and remove themselves from the equation.
I'd like to think that they still exist in the fixed universes, where the multiverse may no longer exist since Element 115 is no longer existent in any time from here on out. To imagine our characters going through their normal lives, having never met, never to deal with the atrocities and horrors they faced in a reality that no longer exists.
It's a sobering thought that they can finally have their [hopefully] happy moments they wished before their sacrifice.
That, to me, is a good ending. Not perfect by any means, but a solid way to close the book for good.