Some people found their niche and created good for the world, for which they were financially rewarded (this stage is every capitalist proponent’s dream).
Then those who earned money this way let their heirs inherit wealth. Some of those heirs multiplied it, and became so wealthy they could influence either politicians or the market itself.
From that moment onwards, generation after generation accumulate power and skew the world to fit their needs, with little regard to anyone else.
Without wealth redistribution, everyone else loses real economic and political power, and this is an inevitable endgame for capitalism because without this incentive such people wouldn’t do good stuff for the world to begin with. We just finally reap the fruits of the system that were always there, and without a great shake, a “communist threat”, this isn’t going anywhere.
The thing that influenced the world and turned the tides in the early XX century was the success of Soviet Union which radicalized people around the globe and forced bourgeoisie to make concessions.
Unfortunately, workers of most countries stopped there, the proposed worldwide revolution never came, and most of the capital remained by capitalists, which has put socialist block at a huge capital disadvantage, with which they still managed to sustain for most of XX century, building huge economies almost from scratch.
Along this way, many regimes have either faced grave mistakes or considered all means worthy, which has led to a lot of suffering. This was exacerbated by the fact that socialist block was primarily dominated by authoritarian regimes, which gave carte blanche for many leaders to act as they please, with little regard to collateral damage. Aside from that, the Cold War, while it can and should be blamed on both sides of the conflict, has led to plenty of proxy wars costing hundreds of thousands of lives.