Wombat said:90+% of society destroyed?
There is no longer a society.
Many indigenous peoples in the Americas would disagree with you there. By contrast, I'd say that the survivors would cling to the values of their culture while exploring how to adapt them to new circumstances.
A magical society's self-sufficiency is a function of the way magic works. If magic requires some sort of quasi-industrial infrastructure, things will get pretty bad. If magic just requires personal dedication, then its masters (and folks with similarly useful skills) will be correspondingly elevated and may spearhead renewal.
People in the culture will reevaluate social arrangements, especially since every worker is now valuable (this is similar to what happened after the Black Death). We in th West tend to have these fantasies about people turning on each other and becomeing savage jerks, but this doesn't actually happen to often. When it does (such as with the Ik, an African community that allegedly became totally amoral in response to poverty), what's more likely is that survivors will think in terms of family, temple and other traditional allegiances and may be reticent to extend full personhood to those outside of it. There are always exceptions, of course, but this is a bit different than magic survivalists.
The one group that is *most* likely to freak out is the privileged group that relies on commerce rather than physical or technical skills (including magic). These groups can't really renew without losing their political power because they no longer have society and abstract commerce to support them. They may try to wrench autonomous groups into provisional governments. If lordship is conferred by the gods, self-sufficient types may have to even babysit this gentry. They have to feed Little Lord Fauntleroy and train descendants to grow up to be more like Aragorn.
I'm pretty skeptical of militaries surviving, simply because the historical record shows that during catastrophes that break basic social arrangements, the military (which in most societies, isn't self-sufficient, as everyone from Sun Tzu onward noted) breaks up into warlord demi-states or backs up one or another former authority. Soldiers are usually terrible farmers and inept smiths because their place is to use the fruits of society to repel that society's enemies.
I *highly* reccomend the novel _A Canticle for Leibowitz_ (and even the posthumous sequel) for anyone who wants to run this kind of game.