You still haven't addressed the core flaw in this argument. If the faction wall is such a barrier to adding shared races, why was the only neutral race in WoW's history added in during a full scale faction war? The faction war is no barrier to neutral races as Pandaren proved, their own philosophical differences saw some attracted to the Alliance and others to the Horde. And it's not as if the opportunity hasn't arisen since.
Both Alliance and Horde adventured with the Nightborne and helped them out, providing an unassailable lore rationale for that particular race to be added as neutral. Blizzard went out of their way to lock the Nightborne to the Horde, I am sure you recall the forums posts complaining about it from a year and a half ago.
And in the next patch we will be both be adventuring with Mecha Gnomes, yet everyone agrees that if they are an Allied race they will be joining the Alliance. How is that even when it can make sense for an Allied race to be neutral they still end up faction locked?
As to the faction war...that isn't the core of my argument at all. The faction war is simply a manifestation of the much deeper factor, the faction divide. The faction divide persists through war and peace, representing the fact that the Alliance and Horde cannot stand each other. The factions are not based on ideology, but on race. The thematic tapestry of each faction is therefore sustained by how these individual races are woven together. And the factions are kept strong by keeping them different, rather than collapsing them into a bland indistinct mishmashes. A state of war or a state of peace is irrelevant, what is crucial is the state of division between them.