By radical left I'm referring to people who run with what Karl Marx or any of his followers preached.
To these people, the only struggle that matters is the one between the rich and the non-rich. Regardless of how well the non-rich are doing, regardless of how much class mobility there is, regardless of how quickly global poverty is being eliminated, the radical left considers the world to be utterly immoral and evil while there is a "bourgeoisie" and a "proletariat."
So no matter how things improve over time in terms of education, life span, earnings, mobility, happiness, etc...the far left is entirely committed to this root principle of "there can't be peace until there is no bourgeoisie." You can look at the last 150 years of progress on all of these fronts across the entire planet but the mountains of evidence will be lost on the radical left.
The people on the far left are always finding ways to spin how things are "worse than ever" or how things are "falling apart" or how things are in dire straits. But they don't actually care about whether or not things are bad and figuring out how we can actually improve things. That is why the radical left looks at millennial earning power compared with housing costs and immediately jumps to whining about "systemic" issues even though they never bother to compare how many square feet of housing people used to live in when you could afford a house on a single salary quite easily. Etc.
Another example: Marxists have been in favor of open borders since Marx's time. This was before global warming was even a blip on anyone's radar. So the narrative today about how climate change necessitates we completely open borders in order to welcome in the impending tide of "climate refugees" is dishonest. (You can find people making this argument here and elsewhere on the internet). It's dishonest because the amount of progress we are making in the fight against climate change will end climate change before it ever gets that bad. Regardless of how much people lie about us being screwed, we will resolve climate change within the next 15-20 years.
The real reason the radical left wants open borders has nothing to do with climate change. They want it because they believe it will destabilize the west and alter the "status quo" away from the "bourgeoisie" and in favor of the "proletariat." The radical leftist will never even consider that we are making progress against climate change and that we will resolve it, because that reality would completely dismantle their previous justification for why we should have open borders. Their intellectual commitment isn't to truth, but to whatever it takes to force us to go down the Marxian rabbit hole. "We have to do this thing because soon the world will end!"
The far left fundamentally chooses its positions based on "does this hurt who I perceive to be the bourgeoisie" in the same way that the alt right chooses its positions based on the equally delusional "does this hurt who I perceive to be of an inferior ethnicity." That's their only heuristic in their decision making process. Neither of them cares about data or evidence. They care about their narrative first and foremost.
If they can weaponize fears about climate change or immigration in order to push people towards communism or fascism, they will. And they must be called out. Neither of these sides is okay with a status quo that isn't their extreme. Even if we were living in a utopia, people indoctrinated into one of these two religions would feel that the world is a terrible place and that they have a mandate to change things. Even if it means being violent.
Be on the look-out for weaponized pessimism.
Mandatory reading: Enlightenment Now by Steven Pinker.