it was made to be implemented to be experienced in-game because of the surprise! Lore the Arbiter become defunct so there is no sorting hat at work.
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population have no meaning when literally a player can have multiple of characters and account. They will be forced to choose if they really wanted to have a taste of playing a race,allied or whatev, and is locked behind a 3rd neutral faction.
Like I said it is an inverse of how Pandaren gain their faction color. You either retain being Horde or Alliance or willfully be part of the third faction by default which is a faction of the dead/undead/Shadowlands.