The Alternate Universe happened. Even though WoD is remembered as the worst expansion in WoW history, we cannot completely ignore it.
The Horde forces have always been less numerous than the Alliance from a Lore perspective. Their numbers must've seriously suffered with all the conflicts happening since the end of the Third War. Legion has been a massive blow to both factions, and I suspect both must've lost close to 50% of their standing armies (speculation) during the war.
With the Highmountain Tauren and Nightborne being ''fresh'' and large populations (comparatively less affected by conflict than the Horde in the past 14 years), the Horde can replenish its numbers quite well.
The same goes with the AU Orcs. The original orcs are a warrior/shamanistic race. Facing challenges is in their blood. Even though the conflict in WoD hit them hard, their population must still be quite high. They are -ripe- for recruitment. AU Grom Hellscream was impressed by the Alliance and the Horde's ''unbreakable will'' towards the end of WoD, and it goes to say Grom respects ''honorable'' strength (despite *cough* being willing to annihilate the local Draenei).
Of course, Grom Hellscream was always impetuous and one for challenge and adventure (MU and AU). After WoD, we can assume he had to ''stabilize'' relations with Durotan and Yrel, given his past mistake of falling for Garrosh's rhetoric (which is a big point of contention for all Lore fans : why wasn't he punished, since his original intent was genocidal towards the Draenei and everyone else who wasn't an orc).
Even then, geopolitics is a game of circumstances. When the situation is somewhat desperate, one can be tempted to turn a blind eye to past history. Sylvanas WANTS more troops, and she isn't one for ''ethics'' in the game of politics. If she can recruit more reliable manpower, she'll do it.
From a gameplay/customization perspective, all you need is the AU orcs really.
1) Their skins seem to allow players to be of every possible clan in AU Draenor. It will allow you to fulfill any orc customization fantasy : want to be Blackhand, Garrosh or Grom Hellscream? Here's your customization, now go farm your mogs!
2) It doesn't matter if they're not from Outland, quite simply because the way they can be customized allows players to look exactly like an Outland Mag'har. You'll easily be able to pretend to be an Outland orc.
3) You can assume many Outland Mag'har have already joined the Horde since TBC. Dranosh Saurfang and Garrosh Hellscream symbolize that. It's just that players couldn't play them yet. You can argue some of Yrel's followers have also joined the Alliance since WoD (it's just they don't have different customization; but as it is, you can pretend to be an AU Draenei).
Bringing Grom Hellscream (and AU Durotan) back is easy with such circumstances, and can provide MANY fanservice moments (Thrall spending time with Grom/Durotan, explaining his universe and the MU versions of their characters to them). But for Yrel, they'd have to bring her back without a ''new'' race, which is a bit more complicated since she doesn't have anything new to bring with her (since MU and AU Draenei are identical from a customization perspective).
With the Highmountain, Nightborne, Zandalari and AU Orcs, the Horde stands to replenish its numbers better than the Alliance I feel. That is because those populations are, in total, higher than the Alliance reinforcements. Lightforged and Void Elves are the main reason for that : they're smaller populations than the Highmountain and Highborne. They're ''elite'' squads of Light and Void mastery, which can compensate their lower numbers. United Dark Iron dwarves and Kul'tiras human will provide more numerous reinforcements to the Alliance.