@jthemangajack I think for the crimson dragon king it's more because of him being a full dragon in mortal and normal human body so it caused a strain on his life force, unlike the dragon warriors who are actual humans but given a drop of blood for superhuman strength. Basically Hiryuu is an atomic bomb (without the power) in the body of an ant, the difference in mass is too great that's why he died early. And he probably also have internal injuries that couldn't heal from the constant wars to create Kouka.
On the other hand, among the current dragon warriors the only one whose clan probably also commit inbreeding will be the Hakuryuu clan, as they pretty much worship the dragons. But since Hakuryuu clan also work as (elusive) wandering merchants, we can assume that the inbreeding may not be as rampant as Hiryuu descendants. Besides, we can see that the birth of dragon warriors seems to be random except for their ancestor being a dragon warrior as we can see from Jae-ha and Shin-ah. Heck, if Kija was not born a dragon warrior, his father would've lived longer until the new dragon warrior is born.