Lore wise, you are 100% correct. That being said though, there has been plenty of times that Blizz has retconned their own lore to justify a game mechanic. When players call them on it, their go to response has always been “a Wizard did it”.
Adding more Race / Class combos could easily be one of those times where it comes down to Gameplay Vs Lore, and Lore looses that fight every time it’s convenient for Blizzard to make the change. Hell, they even retconn their books! Look at the Chronicles; they have been retconned so badly that Blizz had to respond to it, and their excuse was that Chronicles was written by a “Certain point of view”. If that’s not a fancy way of saying that they really don’t give two coppers about maintaining their own lore, I don’t know what is!
Now let me add in another wrinkle; (AND I AM NOT SAYING THAT I WANT THIS TO BE A THING, IT’S JUST A “WHAT IF” SITUATION)
We all know how much Activision likes to monetize anything they can think of, so what if Blizz started offering full Race / Class combo unlocks for $25? Swipe that Credit Card and you can now be anything (except Druids until they make more shape shift forms). Tauren Rogue? Yep. Undead Paladin? You bet. Dranei Warlock? Go Ahead. Bloof Elf Shaman? Do eet.
The point being is that when Blizz wants to change something that goes against their own lore (especially if $$$ is to be made in the process) they have already shown repeatedly that they don’t give a darn about lore and will retconn it in the most lazy way possible, sometimes without even bothering to acknowledge the change “in universe”. Just Bam! “A Wizard did it”.