I think the key factor is keeping the lights on. Without electricity, things would get real ugly, real fast.
Civilization might change in one direction in one metric and the other direction in another metric. For example, if everybody gets richer, but less moral. Has the civilization quotient gone up or down? How do you know whether the population loss has pushed across an arbitrary threshold of “civilization”?
This may well be a minor concern, but at what mortality rate would disposing of the bodies turn into a real problem?
What percentage of Iraqis were killed by the USA? How disfunctional is their society now?
You could even argue that the Black Death had a net positive effect for civilization. The plague caused a labor shortage, leading to higher wages and better conditions for workers, and more social mobility (attempts by the elites to keep the status quo by capping wages would lead to peasant revolts). At the time, Europe was overpopulated and straining its available resources, and a lower population meant a decrease in land prices and better living conditions for those who survived. It presented a huge challenge to people’s religious beliefs and the authority of the Church, leading many down new spiritual paths. It led to an interest in better sanitation and a more sensible understanding of diseases than astrology or black magic.
If you push it far enough, you could say that the Black Death led to (or at least hastened) the end of feudalism, the beginnings of modern capitalism, the Reformation, an increase in secular thought, humanism, the Renaissance, and modern medicine.