Essentially it doesn’t really fit the Alliance narrative (as well as being impossible for gameplay reasons)
Look at the Alliance Leadership:
Anduin: Well he’s never going to allow it.
Tyrande: At this moment I -could- actually see her go for it.
Genn: His beef is with the Forsaken, specifically Sylvanas, I can see him wanting to wipe them out, not so sure of the rest of the Horde(though admittedly he was pro-extermination of the Orcs rather than Camps, but hey, so were the Blood Elves)
Council of the 3 Hammers: Only Moira I can see going for it, to be honest, so she’d be in a minority.
Gelbin Mekkatorque: I Honestly can’t see someone as basically logical and morally based as him, advocating Genocide, especially given the experiences of his people.
Velen: Nope, he ain’t going to allow it at all.
Sure and lets include the Allied Races, Moira I’ve covered,
Jaina: Well, She did try to commit Genocide -Twice- however that was directly after Theramore, I think anyone who has read ‘Tides of War’ can surely agree that she was (justifiably) not in her right mind at the time. She oversaw the pogrom in Dalaran, However: Since then she has proven able to work alongside some members of the Horde (Specifically Lor’themar) and even at the end of the Siege of Orgrimmar she was pushing for the destruction of the Horde as a political entity, not the systematic execution of them all. I don’t know, I think there is enough of old Jaina left, that she would just want the Horde neutralised as a -threat-, and not push for Genocide.
Turalyon: Can’t see it. He’s Lightforged. -Light- He can put up with his wife and her cohorts who are diametrically opposed to what he and his Lightforged stand for, so even taking the ‘Lightforged are religious extremists’ stance, they’re clearly not -that- extremist, else there would be blood on the streets of Stormwind by now. I’d say they are ‘militant’ rather than ‘Extremist’. For him to push for extermination would be very out of character for him, not to mention the schism of faith involved. He would be pushing for the death of people who are faithful in the Light.
I mean of the Horde you’ve -kinda- got Tauren: similar enough to the Faith of Light to be at the Paladin Class Order hall in Legion, even if their faith is technically An’she. You’ve got Goblins, who do -actually- have faith in The Light, it -is- their state religion. You have Blood Elves, now this is the kicker, The Blood Elves, when they were High Elves, worshipped the Light in exactly the same way as Humans. Many lost their faith, but it was rekindled by M’uru, I think it is safe to say there is some element of reverence for the Naaru therefore, I mean Liadrin’s reaction suggests this.
So a People who follow the Light, who followed it the same way Turalyon did, but then were influenced by the Naaru and Draenei influences. I think that would sound familiar enough for him to go “Aww, yeah, That me” to make him abhor the idea.
Alleria: She’d probably go for killing -some- of the Horde, but Genocide, no. Or rather, individual races she might, but not the extermination of the entire Horde, she would not go for that.
So you end up with a weird smorgasbord of views, but the prevailing one is that “No Genocide”. For it to happen, the Alliance itself would have to have a schism and split in two, at which point it becomes a moot point, because half the Alliance versus the entire Horde is not going to end up with the kind of Genocide they were hoping for…
I just can’t see it making sense.