Why are the proletariat a "genuinely revolutionary class"?

The
proletariat is called a "genuinely revolutionary class" because it is the only class
capable of meaningful revolution. Marxism states that in a capitalist society, the only two
tangible classes are the bourgeoisie and the proletariat, as any middle class is destined to
fade into obscurity as the society enters the stages of late capitalism. The proletariat can
function as a revolutionary class due to their sheer number and revolution being in their best
interest.

In a capitalist society, it will inevitably be the contention of
the bourgeoisie that there are no classes and that no issue of...