May I ask you what you think that bargain Zenos struck was really about?
I do want him to come back as some sort of anti-hero. I liked Zenos, and I want things to go forward, but not by removing him from everything and leaving him dead.
Like, I want him to get development. I didn't care about him until Endwalker, but then I was like "huh, he's actually a pretty cool character for an anti-hero rival sort of thing".
I'm pretty sure it was honestly as simple as "I'll help save the universe with your WoL, if you give me a chance to fight him."
As much as I enjoy Zenos as a foil, he's an extremely simple character. He cares for nothing other than bloodshed and fighting someone who could kill him. The fight was really just a kill or be killed moment. And Zenos died. He fought, and he died doing what he loved, and died at the hands of someone whom he thought his equal (or even, at the end, greater than him. I'm sure he knew he was going to die.)
Not going to answer then? That's fine.
Being charitable however, if what you're actually trying to ask if why they get to live on in the story while he doesn't then refer to my first post in this thread that he has served his purpose and can finally rest as a plot device.
I'm pretty sure this does not really apply there : SilverObi was talking about "redemption", not "Extra life for the sake of it".
There's no way, with everything he's done that Zenos could get away like that. He is not redeemable. They already did that with Gaius in a way, although Zenos is yet at another higher level