I think we can assume that Dorgann never let anyone in the crystal/prison world know about Exdeath and how he relates to the crystals just because no one ever comes forward, and there's no NPCs going, "it's just like that warrior said!"
I'm not sure about the timeline, specifically where in Cid rediscovering, reinventing, and reimplementing the amplification technology Dorgann's death falls. It's possible he knew about it, checked it out, and decided that what was happening wasn't a threat to the crystals' integrity, and everything would have been fine if Exdeath's minions hadn't crossed over and started sabotaging the machines.
It's kind of frustrating because the people in Bartz's world were completely caught off guard, and they might not have been if Dorgann had spoken up. Maybe no one would have listened to the weirdo claiming to be from another world, or maybe Exdeath's minions would have overpowered any guards assigned to the crystals, but at least he'd have tried to give them a chance.
It's possible that he thought he'd have more time, but his illness caught him off guard. Maybe he meant to train his son to take his place or to find someone he felt he could trust with knowledge of Exdeath, but his sickness was too severe for him to even pass on information before he was gone.
You're making a ton of assumptions that are just not evidenced and ignoring good reasons for why Dorgann might not act
successfully. Like, going off what's happened thus far, and ignoring if only because blind LP etc any possible future revelations, let's go over this thoroughly.
First of all Dorgann comes from, and is in, a broadly medieval tech level world; the Ancient Ronkans apparently had some snazzy tech, but there's zero sign that anything like a radio exists, let alone a phone.
This means everything has to be delivered
by hand. Messages can only travel by word of mouth or letter carried by a messenger, or, if you're lucky, you have carrier pigeons or suchlike. Even with eg Tycoon having a Wyvern, there's a large degree to which communication is entirely local.
This means that Dorgann
both faced an uphill battle in monitoring- unless explicit evidence of better monitoring being in his possession emerges,
all monitoring he did of both Exdeath's prison and the crystals would be in the form of Go Look At Things And Talk To People Manually- and in communicating. He can't just make an announcement to the world, even making a big announcement somewhere like Karnak wouldn't necessarily spread everywhere.
It could take him
months, realistically (given available travel tech), just to make a circuit of the world and recheck each nation and what not. Days or weeks, at minimum. He can't just call up King Tycoon or whatever even if he's on speaking terms with him, he's got to meet in person. Ditto for if he wants to inform people.
So Dorgann could well be totally oblivious to even obvious if you're looking at it problems through no fault of his own.
Secondly, Dorgann is a dude from another world; for the perspective of the locals, he didn't exist until five seconds ago. Nobody knows his history. He can't get anyone to vouch for him even having existed, let alone having any relevant expertise. This cascades into numerous problems: he's unlikely to get access to anything that the general public lacks access to, so for example the Fire Crystal in Karnak is likely the kind of thing he couldn't even look at. Overcoming any initial suspicion basically would require him to build a life as someone who is not, from the local perspective, deranged and dangerous; it's a medieval world and also a deathworld. Nobody has time for some rando with no history in weird fashion
claiming to be a benevolent hero from another world and warning of great peril, they're busy fighting what, fifty foot long giant killer lobster beasts that sink ships? Whatever they lost Syldra to initially.
And we know that at least Walse and Karnak have prisons. So there's every possibility that trying to tell people about things would just get him locked up and unable to so much as monitor things, nevermind if someone would kill him for being inconvenient or whatever.
But even ignoring the
risk associated with telling people, how do we know he didn't? He arrived twenty years ago. It's a deathworld. He was clearly a secretive man overall, but we don't actually know that he never tried telling people about Exdeath, and frankly he doesn't even have relevant expertise on the crystals we've heard about in the first place. Even if he told
someone, there's no guarantee they'd survive and meet our heroes to so much as remember that he said anything, even if they did remember in the first place; this is clearly a world where people have a limited understanding of the state of their world, the Library of the Ancients is apparently retrieving long lost knowledge that was in fact written down.
So it's entirely possible he
did take the risk, did warn someone and they did believe him... and they just died or whatever.
Like. You're making a lot of assumptions about things that Dorgann definitely did not do simply because we don't have definite proof he did those things and for no deeper reason. It's not
exceptionally likely based on the available info that he did... but how could we tell? There's not actually a lack of evidence that would be a given if he did. Given the conditions, there's simply no evidence that
would be a given, and this is when as I already noted the proposed course of action is, y'know. Risky for him?
Like what good is warning people if you get killed for it and they don't heed or remember your warnings? It's fair to say things would be better if he had successfully warned people, but we simply lack the information to tell whether he was unsuccessful or outright didn't try.