Capitalism is underpinned by precisely two things; the scarcity of time and the scarcity of energy… give the world essentially unlimited energy and you free almost every worker from the pursuit of energy, and now there’s a massive supply of underutilized time. Follow the capitalist impulse to keep the unlimited energy under the control of a cartel and the capitalist(s) discovers right quick what a massive number of people with underutilized time can do with their artificially limited supply of energy (and rough implements) to anyone whose corruption by the profit motive has led them to religiously hate but never really understand Marx.

I wouldn’t want to be a sudden trillionaire (or their armed security) in the face of billions of the suddenly unemployed.

Combine them with the immediate and total collapse of all the world’s petrostates who suddenly have a horde of what is no longer energy but just lubricant and plastic precursor and there goes your world order.

Plenty

or a slight twist, if fusion doesn't bring free energy due to capitalism harvesting a profit maybe THAT is the instigator of its downfall