Can existential crisis wipe out civilisation? Almost yes, it has happened many times in the last century where nations and peoples underwent existential crisis. There have been a few significant examples of the death of big ideas, and this creating destructive downward spirals. I think it's best to base this sort of question in the realities of history. Perhaps the decline of the USSR is a good example to speak of, and you could make something of a sci-fi metaphor of it.
A common view of the end of the cold war is that the Americans forced the Soviets into bankruptcy because of military spending, but the reality is about internal Soviet politics.
The initial optimism of the Russian revolution wouldn't last forever, and by the mid 1970s the country had entered an Era of Stagnation. Mismanagement, economic malaise, and political regression led to widespread disillusionment. A decade later, efforts to modernise and open the system to participation and feedback ended up causing its collapse, because the communist system no longer could enforce the authority required to survive.
Soviet society immediately fractured along tribal lines and split into newly independent republics. This triggered bloody conflicts, from Yugoslavia to Azerbaijan and Chechnya. Corruption has not got better since then, in recent years it has got worse, and Putin's attempt to centralise power has been hampered by growing internal strife.
Chechnya degenerated, and has since then become something of an Islamic Republic. Some of the former Yugoslavia by comparison joined into a new grand vision of a united Europe, and so did better for themselves. But as you'll no doubt have noticed, even that dream is dying, with Brexit, and now the risk of Frexit owing to a possible election win by the National Front.
The thing is, Russia isn't the only example we have of a recent decline. When the Ottoman empire collapsed the Arab world was full of hope about the creation of a union of Arab states which could rival the USA and USSR. But this never happened, regardless of various attempts to join nations together and make the Arab people one. Now the Middle East is a mess, just like the former USSR.
Iraq's fate is very much typical of this; the dream of pan-Arabism died, replaced with a brutal and yet at least secular regime, which itself died (was killed) and then collapsed into an even more corrupt, primitive, and tribal state. Indeed the continent of Africa is full of other examples of fledgling nations full of optimism and hope after independence from their colonial masters, becoming ruined by chaos often caused by external meddling.
So there's plenty of historical examples of advanced societies losing hope and collapsing into bloody war and hopeless corruption during the last century. All you need to do is read history more broadly, mix up examples (Russia, Islamic World, post-colonial Africa, EU), and exaggerate their technological level and the scale of the decline. That should surely provide you with plenty of food for thought!