Thats not true. The high elves left the alliance because they were used as cannon fodder on the front lines. After the scourge destroyed silvermoon, the alliance didn't really much care for the elves (Garithos). The forsaken on the other hand helped the blood elves to fight against the scourge and kill the traitor (Dar'khan Drathir).
They didn't join the horde because of the orcs and trolls but for the forsaken. Furthermore it also depends on the leadership. Lor'themar acts very wise and is making rational decisions. The Orcs under Thrall's command never harmed him or his people, so didn't the Darkspear trolls (who are even fighting against the Amani together with the blood elves). If Orcs would join alliance, Anduin maybe would say "They didn't do anything to us because they are from another timeline!" but I guess Genn would intervene and let his warmongering consultation play on Anduin... Sylvanas accepting humans in the horde? Not gonna happen unless she can raise them to undeads.They joind a faction that were composed of Orcs (they have fought against in wc2 and 3) and Trolls (they have fought against a lot).
If youre talking about WoD then I can say to you that WoD was created under Tom Chilton as Game Director in 2014. Since 2016 Ion Hazzikostas is Game Director and therefore things can change a lot!So this is not something that is entirely new to Blizzard, nor perfectly unthinkable. Blizz was allways a company to do something forcefully different just to be different and and unexpected but yet being totally obvious.
Previous actions show that Blizzard wants to provide race diversification on both sides, I agree on that.We got Voidelves because they wanted to forcefully do soemthing different than high elves, because those were too obvious. Same could happen with Orcs and Humans, so I wouldn't say it is in the realm of impossible.
Personally considering Blizzards previous actions and their mindsets, it is more likely than having Mag Thar within the Horde.
But that doesn't mean that they make up completely irrational things like humans on horde side. For real, where should they come from? What are they expecting from the horde? Why not alliance? There are more reasons playing against than playing for humans in the horde and vice versa with orcs in the alliance.
Like I said, all those expansions were created under another Game Director. Now Ion Hazzikostas is Game director and all the allied races who are known atm are basically same races with different textures (Lightforged Draenei, Highmountain tauren, dark iron dwarves etc.). I guess Nightborne and Void Elves were total exceptions because the squeaking warcry of alliance players demanding high elves in the alliance became too loud and for fairness reason they added the nightborne to the horde (which makes sense if you played through the nightborne campaign).Blizzards seems to love to ad things that are suposedly totally stupid and unsuiting.
We got expansions because of that:
Pandaria, which was seen as unsuiting for WoW.
We got time traveling orcs from a parallel universe ... just to be able to resuse orc warlords.
We even got a cataclysm that did more damage than Sargeras stabbing the world.