Key points Marx made about capitalism's demise include:

1. Overproduction and Crises: Capitalism's need for continuous growth would result in overproduction, leading to economic crises where goods cannot be sold, causing unemployment and instability.

2. Increasing Inequality: As capitalists accumulate wealth, workers' wages remain low, widening the gap between the rich and poor, increasing class tension.

3. Concentration of Capital: Fewer capitalists would control larger portions of wealth, centralizing power and creating social unrest.

4. Class Struggle: Marx believed that the growing suffering of the proletariat would lead to revolutionary consciousness. Workers would unite to overthrow the capitalist system and establish a classless, communist society.

5. Internal Contradictions: The very dynamics of capitalism—such as the pursuit of profit and competition—would eventually undermine its sustainability, leading to its eventual breakdown.

(http://www.autoadmit.com/thread.php?thread_id=5597586&forum_id=2#48107268)