Jack, if you are interested in “what happened” during that 3 or 4 decade period, where the share of wealth of the 1% was much lower, all I can do is give you some starting points. Any of which I am happy to discuss in further depth.
#1: The Purge of the Left: You would need to approach this with a healthy amount of skepticism, but at least It does give you some historical background. I can provide a link to a video Richard Wolff weaves a compelling narrative about the “purge of the left”. Like all good theories it contains significant elements that are verifiably true. And because it’s Richard Wolff he gives the whole thing a Marxist spin, that you probably need to “filter out”.
#2: The Cold War: It wasn’t just about sabre rattling and an arms race. It was also a battle for hearts and minds, a time of unprecedented economic equality in the First World. In 1917 Russia was a poor, undeveloped, agrarian society that could barely feed its own people (not that it really bothered trying). By 1945 in terms of military and industrial strength, and certainly by the early 1950s in technology, the Soviet Union had reached (in around 3 decades) a level of development that it had taken the First World at least several times longer to achieve. This was unprecedented in world history up to that point. You can imagine how terrifying this was to the First World powers. Hence keeping the proletariat in the First World happy was a priority, for obvious reasons. (Note that I am using the outdated term “First World” in its historical context).
#3: I’ll combine a couple of interrelated ideas. Neoliberalism and globalization are probably the big two economic ideologies that have shaken up the world in recent decades. Both had strong support from both “sides” of politics (or what could better be described as “the extreme centre”). Both neoliberalism and globalization could also be simply called “capitalism on steroids”. The best critique of neoliberalism I’ve seen is “Requiem For The American Dream” with Noam Chomsky. Another good one is a written article by Monbiot. Krugman (seen by many as one of the key architects of globalization) has also made a whiny, half-baked admission that he fucked up. That globalisation didn’t work how he thought it would, and destroyed the livelihood of millions of people. Oops. I can provide a link to any of these.
Of course I doubt that you will be interested in any of this, like most of us (myself included) you are looking for something that supports your own narrative. But if you want to discuss further, I am happy to. My understanding of any of this stuff barely scratches the surface, of course.
All of that said, and as good as this video by Monbiot is, I think Monbiot is skirting around one big issue. People, generally, don’t vote according to their interests. They vote according to their values. And you can manipulate people by appealing to their values. I won’t delve further now for fear of becoming the new Al or the new Lestat.
The TLDR version, aka the defeatist version that some ascribe to, is that those few decades of relative equality were a blip. An anomaly. Normal service has been restored. The wealth of the top 1% didn’t go anywhere, governments made them pay more taxes and the labor movement (unions) made them pay fairer wages. But the 1% were still doing fine.