Taking a serious stance here, there’s two types of people who survive the kind of tragedy that the Sin’dorei suffered.
The people who have stockpiles of resources, armed enforcers to protect those stockpiles, and the charisma to keep those enforcers on their side for the duration of the disaster.
And the people who live off the grid and/or are already trained in self-sufficiency and survival outside of the support-structure of civilisation.
The Sin’dorei had both, and ironically enough, many of the Farstriders were Nobility as it was a very prestigious arm of the High Elf armed forces, and after the sacking of Quel’thalas and the ‘destruction’ of the Sunwell, many of the surviving members of the Quel’dorei, now renamed Sin’dorei, would have been responsible for keeping the survivors alive and in relatively good health.
Silvermoon City itself still stood, as a testimony to the determination and skill of the Magisters, Farstriders and the surviving forces of the Quel’dorei army that weren’t on active duty with Kael’thas and/or those that had stepped up from civilian life to be trained by and aid the remaining soldiers in keeping the peace and protecting the people.
It’s always been a theory of mine that most of the Wretched had been, rather than being just the poor, had been the citizens and nobility who had hoarded magical items to sate their addiction to the arcane and had refused to ration themselves. Over-indulging in their addiction without the Sunwell caused a Sin’dorei to become a Wretched faster, from memory, and it could be argued that many of the Wretched we faced in Quel’thalas and the surrounding regions are the result of noble houses falling in upon themselves trying to carry on as ‘normal’ and not realizing that everyone was hoarding magical relics, not just themselves, so when they ran out, there was nothing left to beg, buy or steal, which resulted in the destruction we saw outside of the Dead Scar as riots to get to the magical vaults of specific families tore apart estates and houses only to come across barren and empty vaults.
The return of Kael’thas and the Fel Crystals to sate the Sin’dorei’s hunger for magic also played a part in allowing Silvermoon to remain functional, and I do not doubt that Magisters would have, before then, been fighting their addiction to charge crystals of their own to hand out to the civilian population to sate their hunger, which could also have explained why only so much of Quel’thalas was actually rebuilt, because the Sin’dorei Magisters were spending all their time and effort keeping their peoples’ addiction in control while the Farstriders, and later the Blood Knights to a lesser extent, were spending their time keeping the Amani Trolls away and keeping their people fed and sheltered.