By that argument, Blizzard should never introduce a new character, or bring one out of the background. Varian’s dissappearance and return was a key element of the Onyxia back story.
Nathanos again is a major part of Sylvannas’ story, as the only person that Sylvannas really seems to trust his placement in the Sylvannas storyline makes sense. And he does have provenance as an outdoor raid boss that would frequently lead to world PVP back in the old days.
Both of these characters pretty much date back tot he beginning of the present game and beyond.