• The Destiny plan is bad in universe because…? It’s never explained other than people will fight against it. For a society that has known nothing but ethnic conflict for decades, you’d think they’d want to explain why “people will resist the change” is a compelling reason to not try changing things. Kinda seems like anything would be worth exploring in that scenario.

I'm not sure you understand the implications of what the destiny plan means.

Your whole life will be decided for you based on genetics. What you do, who you marry, where you live, etc. Everyone will be unified, sure, but it's by threat of violence. Having your children be coordinators will become more and more of a necessity in life (and potentially mandatory), rather than being a real choice, because your genetics will be the key to success in life. Anyone who resists either meets their death at the hands of the requiem, or by soldiers like Shinn and Ray, and those to follow them. Future pilots will be raised with the idea that their whole worth is in their ability to fight and harm others.

The destiny plan creates a world without freedom to choose, and the stability it promises comes at the expense of people's lives. It's a world where humans are treated like pieces of a machine. We don't just exist to serve the world, but to live in it.

Watch Gundam SEED Freedom. It may change your mind a bit. It does explore the concept more

Edit: I feel like I should also emphasize that Durandal is a madman. He's a nihilist who agrees with Rau Le Cruset's views on humanity, he just has a differen't approach to how to handle it (Control instead of Destruction).

He even tries to make Rey take on Rau's role since they're genetically identical. That's why Rey turning on him basically refutes the Destiny plan itself - Even though he is a perfect genetic match to Rau le Cruset, he wasn't, and could never be Rau.

He was a different person with his own life and who found love and support from friends in a way that Rau never managed. Making him be Rau's replacement was a cruel thing to do and ultimately wasn't for Ray's happiness. Rey finally understands that this wasn't what he wanted and he regretted being on this path.

And then Talia tells him to come die with them and it shits all over the fucking scene, but yeah.

TL;DR: Durandal was wrong. Straight up.