Even as one of the resident story forums night elf fans, I’ve never really been one to consider the Burning to be one that wiped out the majority of night elves, not even taking into consideration the entirety of the War of Thorns. We’re never given any set numbers beyond what the one quest offers.
However…
Assuming that the 900+ civilians dying as shown in the final quest is accurate, and taking into account however many died in the war itself, we can get a number of maybe 1200 dead night elves. Old lore stated that the population of Darnassus/Teldrassil was around 15k, and if the majority of night elves lived there, we can generously say that maybe 14k more lived elsewhere. So at minimum we’re still looking at about 4% of an entire race killed. For the sake of a plan that wouldn’t have worked even if the Burning hadn’t happened.
Other races have suffered far larger percentages of losses, but this was done by the Horde, a player faction, one where the logical repercussions were it an NPC threat can’t apply. And it was done, narratively, in such a way as to have it get swept away within less than a year for the sake of moving the story along for the same characters to make peace.
At this point, it’s not even about revenge, for me. It instead showcases a very odd disdain on the part of the writers for night elves, and draenei as well, and a penchant for letting races they favour run roughshod over the ones they can’t write. This has been a thing since Cataclysm, and the more it’s gone on the more the night elves have gone from feral, savage people to more typical fantasy elves.
That’s why the Night Warrior stuff annoyed me, to an extent, because… it was basically just getting night elves to where they already should be, and have been. But yeah, night elves actually getting revenge was never going to be on the table. The writers wanted a big, badass metal moment for their preferred vision of the Horde, and then backtracked so that they wouldn’t need to have something equitable happen. Which is easy when they frame any retaliation by the Alliance, however measured, as equally bad.