It's trivial to justify most things within the in-game lore. Used to be only NElfs can be druids in the lore. Heck, used to be only MALE NElfs can be druids. That was obviously changed rather quickly. NPCs broke the mold fairly quickly. Now it's expanded to all sorts of races. NElfs also couldn't be mages for a pretty significant lore reason, but that changed - the sky didn't fall, and to no one's surprise people did NOT suddenly go "a friggin NIGHT ELF mage just crossed my path in Stormwind, my immersion is now RUINED FOREVER and the ENTIRE GAME is broken beyond repair, I'll unsub and quit immediately HOW DARE THEY" all over the place.
No one is asking for anything that others can't do already, like if people suddenly went "but I want to play a half-Tauren-half-Gnome Battle Barista that's also 16 ft tall because of a hormonal disorder and has completely neon-green fur because of a dye-factory accident when they were seven" then I'd be more onboard with a complaint, but just allowing people to choose a race that already exists and a class that already exists is not going to upset ANYTHING of any relevance whatsoever.