Blizzard really wanted the War of Thorns to have an emotional punch to it, and flipping it this way wouldn’t have that effect, I don’t think. Sacking the Undercity would read as “justified” to a broader range of people, whether because the villainous Forsaken were brought to justice or the pitiable ones were given a mercy killing.
Now as a player I’d find that really interesting because that’s a lot to unpack, but the official story wouldn’t dig any deeper because that’s just not what the writers do. But I also think their attempts to get serious and real about their Very Silly Setting rarely work (mainly because they’re so surface-level about it) so on the whole I think this reverse scenario would have been better.
#LetTheTreeElvesBecomeDeadElves
Yeah, I dig it. I’d also like the reason they marched on Undercity to be something to do with, you know, AZERITE.
And maybe Tyrande is already the night warrior, she’s powered up to safeguard her mind from Azerite’s effects but the power up has its own consequences. Though I wouldn’t to strip her of her agency so I’d be careful with that.
Side note, I was farming to build up more buy-ins to horrific Stormwind after the reset (I’m farming the toy vendor for his toy, no luck yet, and also for the weapon enchant illusion, no luck yet) – anyhow, I was struck by that feeling of “how did we get here?”
It just seems like such a clumsy take on narrative design, it’s so bad that we’re spending 10ish months or more playing RNG slot machine games with the old gods. That I’m in Pandaria and Uldum of all places, gathering currency to clear all 5 wings of the thing over and over again, mostly for nothing.
The faction war expac, following your idea, should also just play out way different. I’d even leave Azshara out of it. And the old gods. Bring them in for the next expac IMO.
This reminds me of going for a mid-level job and you suddenly find yourself as the one in charge of the entire place as the big boss goes off and does all sorts of wild stuff. Next thing you know, two burly dudes are holding some debtor’s arm under an industrial cutting machine and you’re like “if you’re going to cut his arm off just f’ing do it already, my guys need that machine!” – you check your watch and it’s 2:30am. You’re sleeping there again, aren’t you? And you literally use a plastic bag full of your week’s bonus money as a pillow.
“How did I get here?”
Yet for some reason, quitting WoW is harder than quitting that job 
So, because it randomly for whatever reason came to me at work…
imagine my post, but Tyrande doesn’t do the burning, Sylvanas loyalists who arrive to the battle conviently late do (and promptly disappear as it ends) do the burning of Teldrassil to ensure she’s actually villainous and ensuring the war escalates. This also being how as things begin coming to light, Horde has cause to view her as evil rather than someone who went “'Ight, imma head out.” at the opening bell.