You keep mentioning Cordana but let me clarify something: Cordana was NOT an Alliance character in WoD. She was a neutral NPC who was part of the Draenor expedition and she worked alongside Khadgar the whole time, Horde and Alliance both got to see her complete arc to betraying Khadgar at the end, and she was never really the focus of any of it. So she went from Neutral NPC with little development or backstory to a dungeon boss.
I’ll try to keep the Horde significance short, but in a nutshell here is why the above mentioned characters are important:
Cairne: WC3 hero, founding member of the New Horde. Horribly under utilized in WoW, fleshed out just enough to be killed off-screen by Garrosh and Magatha Grimtotem. Cairne and Thrall were extremely close.
Vol’jin: Another founding member of the New Horde, heavy development during MoP, primary protag of a novel and several short stories that flesh him out. He was an exciting Warchief pick because he was such a cool character with the right intentions for the Horde, only to be killed off after all of his development and replaced with something that has splintered the Horde fans and characters.
Saurfang Jr: Introduced as one of the first major “Pure Orcs” in BC, alongside Garrosh. He was the perfect Orc to establish an arc about embracing honor and resisting the bloodlust that Varok tried to protect him from, that arc never actually got explored but a lot of players wanted to see. A LOT of people were speculating that he would be a potential big-name hero to replace his aging father, same for Garrosh, instead he was killed off.
And the really irritating thing isn’t that the Horde heroes are dying so often, but that they keep screwing with the one they name as Warchief. We keep losing our leadership for stupid plot twist reasons that end up making the Horde feel worse by jerking our story line back and forth between noble savagery and honor vs cruelty and bloodlust war machine.
Garrosh could have been a freaking AWESOME Warchief, they go as far to admit it in game. A bronze dragon says something like “Our version of Garrosh was one of the worst possible, in many other time lines he is regarded as the greatest Warchief that the Horde would ever know.” That alone was infuriating to me.
Then Vol’jin gets his shining moment in MoP. We have a founding member as Warchief again. He is honorable, he is strong, but he is also cunning and ruthless. Fans were pretty receptive and thought we had a cool story coming. He gets offed quickly after being benched for most of WoD and then we get Sylvanas, who has not been popular or good for the story, just another murdering psychopath like Garrosh who is barrelling straight to another civil war story.
So no, Cordana Felsong and Sira Moonwarden do not hold up against our characters that have mostly been around since WC3 and only developed to be betrayers or casualties. Plus, we keep losing faction leaders and they never get replaced. Right now the Orcs and Trolls have no declared leader and the Forsaken actually had the other half of their leadership killed by Sylvanas in Before the Storm.
The one Alliance death that I will grant, who everyone always fails to bring up, is Maraad. He gets great development in WoD only to be killed off as a martyr for Mary Sue Yrel. That was the Alliance instance of introduce a character, make him cool, kill him off as a plot springboard.