No you don't understand, he's misunderstood. I can fix him! Ugh.
ForteNightshade basically said everything I wanted to say before I had a chance to post.
In order for a redemption arc to work, the character has to have redeeming qualities. Zenos does not have these. He was willing to usher in the Final Days and destroy the entire world for the sole purpose of making us “hungry enough” to want to fight him as we did at the end of Stormblood. Not to mention he got upset when he ran into us in Garlemald and we were focused on something far more important that giving him what he wanted. He’s a selfish individual with no redeeming qualities; and a redemption arc would be poor writing. And, frankly, an insult to Natsuko should she attempt it. She’s far better than that, as Shadowbringers showed us.
Any potential the writers were going to give him should have been given in 4.x. But they didn’t. They made him a boring and uninteresting “main antagonist” that a lot of people are fine to see gone. This send off was the best they could give him. Leave it at that.
Not every villain is an Emet-Selch, nor should they be. Zenos was properly developed. He stated his reason(s) clearly time and time again. This is who he is. A redemption arc wouldn't really work with his character, and I like him as is anyway. Zenos wanted the ending he got.
While you're not wrong, he did develop ever so slightly. What Alisae said to him struck a chord. A lot of the things he did was in order to get the WoL's attention, but he finally realized that wouldn't work. And in the end, Zenos even gave WoL the option to reject his challenge. So he did change a little. It may seem insignificant, but to Zenos, that was his only joy in life, and ordinarily, he would stop at nothing to attain that joy. Yet despite how much this meant to him, he would have let the WoL walk away if they wanted to.
I'm not saying he should or should not be redeemed. I'm just saying he did develop to some (small) extent.
I'm pretty sure Zenos is one of those characters/products conceived by the marketing department to serve as a counterpart to Y'shtola in crossover games/media like Dissidia,etc. Unfortunately rather than making him an ultra edgy anime antagonist like Sephirot they made him more like that creepy guy at work that stands way too close to you in the elevator and says stuff like "Your hair smells nice" in creep like fashion. Probably sifts through your trash when no one is looking.
Then the story completely negated the development you pointed out. What would be the point of the development? The point of the choice? You can’t say no—so the “development” led nowhere. Effectively undermining it.
It would been much better for the game to break away from the prior notion of choice not affecting the story. And would have probably helped what you’ve pointed out here. But the fact that it undermines it doesn’t help Zenos at all. It makes the entire thing worse, actually.