This is mostly just gameplay. The narrative makes clear that the majority of the player races stuck with Sylvanas, with Saurfang's fireplace gathering being described as being 'so few' whereas Sylvanas is described as growing in power every day. Bob mentions that his people would sooner support her than him, possibly because he sent the Horde PC to kill guys who survived the Purge of Dalaran on behalf of Baine. In the same scenario you see orcs willing to kill Green Jesus. Throughout the story you see no large-scale resistance to Sylvanas like you do to Garrosh. The narrative both shows and tells you repeatedly that her approval rating is far higher than Sadfang's. As for the Forsaken, they have all the ones the Scourge didn't raise to work with in terms of Lordaeron's dead population and that crappy tie-in book tells us Sylvanas also has a policy of raising her troops when fallen. If any race should increase instead of sink in population, the dwarves are in the first post since they united all the clans with no real losses to themselves and second should be the Forsaken since they're not bound by birth rate and grow with victories without being too hindered by defeat.
Would it have made more sense if they were fewer than they were? Sure, given that one patch ago the Alliance was apparently weeks from victory. But that's not the story as written. After all just one patch before that Anduin was so strapped for troops that he was calling up farmers and he was sure he couldn't win without Sadfang's help.
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Feanoro
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Val the Moofia Boss' analysis is mostly bs. It shills for things like Stormwind's industrial potential, this being the place that's received the largest nonsensical power boost, ahead of even the orcs. In the First War the Horde kill most of the population that couldn't hide in the sewers or escaped on a few boats, with those survivors migrating to Lordaeron which was then killed by the Scourge/got mass raised by the Forsaken. Per Chronicle they were also previously almost overcome by gnolls, a meme race, and Gurubashi remnants, needing Medivh to save them, this being the equivalent of the kind of headhunters that run into the Iron Juggernaut's sawblades in SoO. It also wanks for the spaceships who's finest examples like the Xenedar gets oneshot by a nameless demon in its very first appearance and who aren't even produced by the draenei but are charity by the naaru. This is a race that was at serious risk of destruction by a fraction of Kael'thas' forces, who were themselves a fraction of the blood elves, who are themselves a mere percentage of the pre-Third War high elves.