In the Communist Manifesto, why did Marx and Engels think that victory over the bourgeoisie by the proletariat was inevitible?

Marx and
Engels believe that the victory of the proletariat over the bourgeoisie is inevitable for two
related reasons.  First, they say that the capitalist system makes it impossible for the
proletariat to continue to exist.  Second, they say that capitalism forces workers to come
together and unite, thus ensuring that they will defeat the bourgeoisie.

First, the capitalist system makes it impossible for the proletariat to exist on its
own...