He was still very much demonic and "insane" by the end of the game. I don't think he was good--he was definitely evil, but there absolutely had to be something good about him at one point because he did make a massive sacrifice and was then cast out for making said sacrifice. But I don't think it's too large of a leap to say that he knew precisely what would happen if he "lost," and by letting Noctis grow to full strength he absolutely creates a scenario where he can lose. Maybe it wasn't his total intent, but if he just wanted the Plague of Stars to happen he could've just off'd Noct right then and there instead of setting things up so the Plague can be cured--which is what his original intention was back before he was demonized. I dunno, I think there's enough depth to draw various conclusions about Ardyn's "true intent" and while I wish there was more I'm also happy it wasn't a pure exposition dump about his motivation.