my dude, you are the one ignoring the lore.
you are literally ignoring word of god by blizzard, saying without horde and alliance the legion wins, the only way to defeat the legion is with both factions and without horde there is no alliance.
think geco, think!
the amount of presumption and headcanon here is giganticThere is no legion in W3. Tamponco can come in Bc. nor in Legion.
Sargeras exists but there keep looking for how to enter.
And when he enters Azeroth he will have the strength to fight.
I mean, how much do we need to finally avenge the Legion?
You are trying to paint a picture that since things would go different, because no orcs, it would mean one, the legion would not come here ever, and two, we for some reason would just work out to be ready when the legion arrive.
That is just wishful thinking, assuming every single thing would go in your favor.
Without the orcs, the Legion would come here anyway, one way or another
And when they arrived, there would be no alliance or horde, therefore, the Legion wins. Again, the legion can only be defeated if Horde and alliance exist, ebcause the devs(word fo god) said so. that is not hard to grasp