Marx argued that capitalism had built in contradictions that would eventually make it unsustainable. He basically predicted capitalism would collapse under its own weight, but didn't specify how long it would take to finish dying. Things like falling rates of profit, ever worsening crises, and the immiseration of the working class are all precisely the things we see unfolding around us today. At some point all these problems must reach a breaking point where the system as a whole can no longer function.

Arguably, Marx underestimated the ability of the capitalist system to adapt and it ultimately proved more flexible at kicking the can down the road than he gave it credit for. Marx also thought the crisis would hit the most advanced industrial nations such as Britain and Germany first. Instead, capitalism fueled imperialism exporting its worst exploitation to the Global South. The core countries got to enjoy relative stability partly because the worst human abuses and extraction happen elsewhere.

Lenin followed Marx's logic and argued that Western capitalists used superprofits from colonies to bribe their own working class with higher wages to buy off revolutionary potential. Basically, the system managed to survive this long by making the Western working class comfortable enough to not want to burn it all down.