Civilization isn't going anywhere. The current regimes that govern the world may be unstable, but urbanization and technology has been reinforced so much that it is unlikely that industrialism and its technologies would be lost.
After the Roman empire fell, people did not loose the knowledge or skills of the Romans they lost the ability to organize the amount of people necessary for using it.
The reality is that the current states of the world have planned around a nuclear war and have already decided who gets to live and who gets to die, and know full well that once a nuclear exchange begins, enough of those chosen to survive will be secure in bunkers. Those who were not chosen who survive both the nuclear war and starvation will be rapidly subjugated and either exterminated or put into chattel slavery for perpetuity, as will their children and their childrens' children.
Science and technology won out because it enabled better quality of life, without that it'll likely be seen as whip and fade.
Chances of retaining organized civilisation beyond a nuclear war are very low, while there might be a chance that the planet remains habitable, however most industrial capacity will get wiped out. And your James-bond villan fantasy of cleansing the hordes and then reseeding the planet with the chosen ones, is a unrealistic phoenix from the ashes mental masturbation.
I don't know how anyone believes we can keep going the way we have since the 1990s and not be in full Nazi mode by 2050 at the latest, and judging by the prevailing attitudes in America they're trying to push full Nazification by 2030.
Well Nazism with American characteristics is going to reduce it to medium regional power, and Europe will be reduced to a appendage of Eurasia.
As for global warming and crop failure… that isn't even going to kill 10-20% of the population most likely, and all of the burden for climate change will be pushed onto the poorest and the unwanted in society. The starvation could be averted with sound management and planned economy, but no one in power wants that, so the effects of climate change on the underclasses of society will be deliberately exacerbated.
Finally the left has found their revolutionary subject
Far from "destroying civilization", climate change will strengthen the strongest regimes of the world by reinforcing their ideology of ultraviolence and competition for competitions' sake, especially the United States whose populace is so beaten down and brainwashed that they can't even consider protest let alone revolt.
Nonsense the most reasonable societies that engage in long term planning are going to be the ones with the best chances of survival, for example the more renewable energy you build the less you will be subject to coercion, via stuff like oil dependency, you will not need to engage in pipeline wars… If you transformed agriculture to become sustainable, you will have reliable harvests, and will not have to deal with enormous buffer stocks, to avoid crippling famines… Hyper competitive systems cannot do long term investment, and will look really old in few decades compared to long term oriented societies reaping the rewards.