Ideas don't stay discredited forever

Your thoughts here all seem plausible, but I think perception of corruption and disappointment with economic growth and related standards of living are also important, along with social and technological change that are occurring too rapidly for some.

First, the stable perception of democracy and capitalism seems to be: I want it but not too much. I think liberal democracies are in the “not too much” phase today but I’m not worried. Democracy and capitalism have a high approval floor because they bring freedom and wealth. But their approval ceiling is limited because people are too anxious and short-sighted for democracy (this problem is too scary to wait for consensus) and too jealous for capitalism (poor are stealing our jobs and wealthy are stealing our capital).

Second, it’s harder today to “solve a problem” due to the cultural fragmentation of the world’s population due to the internet. People find niche groups that appeal to them, are bombarded with consistent messaging and data overload. They see and hear their neighbors and make enemies. They are so radicalized that their minds become closed. And they see everything their government representatives are doing and demand blood now, where in the past those representatives may have had more of a leash.

I do think there’s hope in the internet age because radical transparency shows neighboring constituencies which policies work and which don’t (see police reform, drug legalization).

Third, governments decay over time. Corruption is quiet to begin and noisy to take away. This is where I think lessons are relearned over decades or centuries.

PS: Defense treaties seem great when there is one dominant group. When there are equally powered groups, defense treaties seem unstable and risk amplifying, aka WWI.

Somehow I can’t believe the near-global degradation of politics and public policy is about the waning protective effect of ‘lessons’ that seem moderately specific to particular countries or continents. For example, India didn’t experience the world wars on its home soil and yet stayed away from authoritarian nationalism until fairly recently. Meanwhile, Russia had barely 20 years’ break from Soviet socialism before opting for a return to something at least a little similar. To a lesser degree, the same goes for China. And for lessons to be imparted, people need to be educated about them if they did not experience them first-hand; but as you (and Bryan Caplan) say, most people don’t learn or remember much from their formal educations.
Another potential explanation is that the global financial crisis undermined popular belief in the legitimacy of liberal capitalism. But outside the US and Western Europe,  the GFC was actually not that big a deal. And even in those regions, what resulted was a severe recession (rather than depression) although not that much more severe than the recession of the early 1980s, which if anything led to the speeding-up of market-orientated reforms. Maybe the recession was longer and unemployment stayed higher than in the early 1980s, but not dramatically so. 
I think there is something else going on, and it is hard to avoid thinking – sadly – that it has something to do with the death of expertise and the democratisation of voice. I think the global warming debate was one of the first victims of this shift. Perhaps I erred in reading Bjorn Lomborg fairly early in the piece; and I understand that many scientists believe his climate-related claims are wrong or at best misleading. But at least at the time, what I seemed to observe was that although global warming science made intuitive sense, prominent scientific institutions and environmental scientists took it upon themselves to exaggerate the harms, underplay the perhaps only medium-term benefits, and dismiss (albeit unpopular) solutions like nuclear power. That led to me as a non-crazy person to question their good faith and credibility. Once that trust is lost, people start to wonder whether everything else they’ve been told to believe is good for them, like markets and trade and immigration and meritocracy – and vaccines – really is so. Like you said in a previous post, experts and governments should just stop fibbing to people to manipulate them to do things. If and when those in authority do fib, it doesn’t take much searching on the internet or social media to find someone who can make a respectable case that readers are not getting the whole truth. Once expertise receives no respect, people flock to simple ideas, quick payoffs and tribalism.

No, they don’t stay discredited forever.  You have to be pushing 50, for example, to have much of a memory of the cold war, the fall of the Berlin Wall, and Tiananmen Square.  Hitler and the Nazis, of course, are older still, but culture does a reasonable job of keeping them and their crimes in public consciousness (though less so for Stalin and Mao unfortunately).
Both nationalism (go team!) and communism (it’s just caring and sharing!) will always have superficial appeal to the human psyche.  As living memory of the downsides gradually disappears, the risk of recurrence rises.
 

We live in a predatory society.  The American Bankers Association paid $3,000 to economists not to debate the issue of time deposit banking in 1961.  Government is run by the lobbies. 

Apparently another idea that hasn’t stayed discredited is technocracy, as you advocate it as not only a, but the alternative to authoritarianism.