Yeah but the supposed wise beyond his years 18 year old King led an Army into a fight at Undercity with no Gasmasks, was he wanting to be the next Bolvar? Then he walks up to Sylvanas and tries to arrest her for genocide. When it’s plain as day you’ve just walked half the leadership into a deathtrap.
Blizzard isn’t interested in writing Alliance storylines with interfactional conflict to explain why they don’t simply focus all their might on destroying the Horde. Instead they simply stupify the Alliance down to the level of infants.
Alliance doesn’t kill Horde because they are all morons. That’s the story reason Blizzard goes with.