Over simplistic view of class - ignores many other power imbalances such as sexism, boss/employer, racism, homophobia etc. More than 2 classes - under class, working class, middle class, upper middle class, upper class, aristocracy etc.
MARXIST RESPONSE: middle class are still proletariat if they have to sell their labour, still alienated as their life is controlled by bourgeoisie capitalism.
The only way to end all other imbalances is to end the class struggle, if proletariat are not alienated through capitalism they will not take prejudices out on each other. Marx argues divisions are created by the bourgeoisie to distract the proletariat from capitalist exploitation.