I think a lot of people overlook how damaging losing Teldrassil actually is.
It is not just the destruction of the nelf capital, it is not just the genocide of nelf people, but it is also the destruction of every nelf achievement since Classic. From Classic to Cata, the entire Nelf story was about the purification of nelf land in the aftermath of the Burning Legion’s invasion. Another reason why it is bollocks to say the invasion during Legion was the largest demon invasion ever, because the environmental impact is not even remotely as devastating as Warcraft 3.
Back on topic, Teldrassil was symbolic. Lacking the blessings of the Dragonflights, the Night Elves struggled with the new world tree, which was corrupted by Xavius. But through their faith and perseverance, and staying true to their age-old convictions, learned from for mistakes of their bloody history, the Kaldorei purified the tree with the end of this questchain being “Vengeance of Elune” (Sound familiar).
Impressed that Teldrassil can prosper without the influence of the Dragon’s blessings, Alexstraza and Ysera blessed Teldrassil, once again returning strength, vitality and a connection to the Emerald Dream. This marked the dawn of a new and more prosperous age of the Kaldorei… Until BFA.
In a blaze the likes of which Azeroth never seen before, all of that is now gone. It is unclear if the Aspect’s blessing remains or not, but assuming that as incinerated by the flames of the Horde, the plight of the Kaldorei becomes more than just this single tragedy. A vulnerability to disease and a severed connection from the Emerald Dream.
Teldrassil was everything. Everything the Kaldorei had done or ever will do. Everything the Kaldorei player has ever done since classic died with Teldrassil.