#tbh im not really a jaeha expert so if im wrong feel free to correct me...

[soo-won]

The real tragedy with Jaeha to me, outside of the recent events, is his realization that he's losing his usefulness for yona, and even starts to chain her down. I think. This is all in my head for now truly but I'm like,, damn he sure is taking the L a lot recently! so I pray this will be adressed later on but you know at the beginning of the castle arc when he tells Yona hey I told you I was your legs :) and she rejects him, because she can't risk it. Like, Yona wasn't in the wrong here, it's just that Yona in the castle arc was entering a battle that has no need for the dragons' powers. People in the castle arent chained literally, they're chained by their duties and the isolation that comes with having more power and information. That distance is not something that can be crossed literally by flying away. But anyway it was still painful for both of them, Jaeha quickly connects the dots and understands the reason behind that, but that only makes him more willing to find a way to help.

Then he is deployed in Kin Province, and as he realizes that Hak is fighting there he makes it his new goal to bring him to Suwon, only to fail to save him from the flooding, as he sees him be engulfed in that wave under his very eyes, and can't find any trace of him for days after it happens. Losing Hak is devastating enough given how much he cares for him, but it adds to the hypothetical feeling that he failed and his leg couldn't bring him to Hak in time.

But it's fine, his presence alone is enough, he is loved and he's like a big brother to Yona, if she needs comfort, because hell this is about hak obviously she will be devastated, he can comfort her...and then Yona hides her sorrow from him and confides to Suwon of all people instead. Sure breaking down in front of Suwon wasn't her intent at all, but she still did in the end. Yona used to cry and confides to Jaeha before (like in the Sei forts arc when she failed to protect Lili), so why? And how not to be jealous of Suwon that seems to have a bond with her that even death and betrayal will never completely break?

He plays an important role in the battle against Kai, so sure, at least his powers still have some use and raises the moral of the troops as well, but now these same powers causes terror instead.

And before that he got captured, again, and as much as Yona defied our expectations and showed her development with a cycle of similar events but a better outcome and way of dealing with it (ie: kidnapping scenario), that is not the case for the dragons. Yona had complete control over her capture, it was her plan, and she could free herself with her own power. Yona is moving ahead of them, never intending to leave them behind, but she doesn't depend on their powers at all anymore.

So really, what does Jaeha have left at this point? Now Yona is the one who has to save him from his own powers! Jaeha who refused to join Yona at first because he didn't want to be chained down by destiny and a prophecy is now the one chained her down oh the irony of it all...

It's no surprise that Jaeha would decide to leave the ddhhb for good, his legs literally can't bring him to them anymore and worse he risks to hurt them, no matter how much he wants to come home, to them, he just can't. He can't be what Garou was to him and selfishly weights them down.

Since the beginning of the castle arc he sees again and again that Yona and Hak are doing fine without him. If Yona is in a pitch or needs comfort there are other people she can seek out. Jaeha wanted to help Hak reach Suwon and in the end he did it all on his own (Just to be clear Hak could obviously not have emotionally done it without Jaeha's support). The encampment arc ended up being resolved by everyone but the dragons, the story in general is at a point where their utility reached their limit, so what now?

The crimson illness is very tied to the mental state of those who have it, worsening when they're under stress and emotional turmoil, so I don't think it's too farfetched to theorize it might be the same for the dragons now. Kija is able to resist it because he has always been the most strong willed and stable of them. So yeah tbh more than Jaeha dying to me I think it's more him isolating himself to protect yona&co and he's not done done for. for now. I don't think he's lying about reaching the end of his lifespan etc(it's the same word as the end of an object's useful life in japanese btw.) but anyway oh well. All that to say I don't think he's dead and this is very much the occasion to finally break the connection the dragons have with their tragic fate and their powers that bring far more bad than good. What Jaeha needs is the reassurance that he is needed and that this does not depend on his powers at all.