They're not doing this with current upcoming race customization. And nothing they've said indicates this. You'd be absurd to think that the amount of variety the original races are getting won't be something desired by those that consider an Allied Race their favorite race to play.
Also here:
"Want some of my jungle love?" - Male Troll flirt. Specific to the jungle.
https://wow.gamepedia.com/Flirt
Again, not that it even has to go that deep because a 1 population race (Dark Troll) is becoming a customization option.
Everything about the upcoming customizations just screams "variety" "diversity" and I think you're at a fault trying to lean in and think that people who unlocked Allied Races would like to be treated 2nd-handed in terms of the increased customization options.
These are also races that are considered "prestige races" that you
unlock aka more effort put into getting to play them than any of the current original races. Why would people who put more effort into a race unlock be alright with lesser diversity in their race options when their time comes?