I don’t think that would be the case. There is a lot more friction within the Horde than within the Alliance imo.
Tauren vs. Goblins are dimetrically opposed to each other and the Forsaken pissed nearly everyone off as did the Orks with their Garrosh episode. I do not see that, without gameplay restrictions, the Tauren would still join the Horde when another war happens (especially with the Horde being the aggressor again) and Forsaken and Orks only probably put up with each other because without them they would be militarily insignificant.
The Alliance also has friction but I don’t think they are as big as within the Horde.
Is it really trust to believe that someone won’t do a thing they’re not even remotely capable of? Like i know that in WoW Blizz is horrific at keeping track of their numbers, but Darkspear and the Tauren are nearly extinct and the Kalimdor Orcs all fit on a few stolen ships so if anything the harder sell is the Forsaken believing they need them.
I’d call it not unrealistic enough to take you out of the suspension of disbelief so it might not be pretty, but does the job it’s meant to.
And nobody did besides the Blood Elves, for very specific reasons.
Cleansed after the death of Mannoroth
Different trolls, who are still hostile to this day.
That’s a thing?
There is a distinct line within the Horde which way it would have split. Blizzard has put a few too many eggs into Orgrimmar, for gameplay reasons since then and has recently been on a D&I crusade, but at least the Forsaken have their gloomy forests and stuff back. And as i said sometimes the level of trust needed for an aliance is knowing that if you’re attacked your attacker is going to be attacked by your allies.
The thing is, as much sympathy there is between Tauren and Kaldorei, as much animosity is there between the Orcs and the Kaldorei. Because the Orcs cut down whole segments of Ashenvale and killed Cenarius while on the other side the Orcs saved the tauren from extinction when the Night elves decide to tell them they should take care of their own problems.
So if there would had no need for two factions gameplay wise then the story would be totally different. In this timeline than after the invasion of Daelin Proudmoure, the battle of the Hyjal would be an incentive to form a union between Theramore, Ashenvale and the Horde while Stormwind and the dwarfs would be vary of them. The Forsaken would still do their own thing and either get connections to Stormwind as their dead thought breathren that now are a bit different but as Stormwind had back then so many other problems they would not have seen in person what the scourge done there would be another kind of Alliance.
Or as the humans despise undead from their look alone there would be a rift and the forsaken stand on their own. Maybe instead Illidan would had success with his plans and so instead there is a third faction based on the naga, the Blood Elves, the forsaken and the Draenei.