Critics of the wealth gap might argue that

The economy will eventually stagnate or decline.

A large wealth gap creates a lot of wealth at the top and very little at the lowest levels. The middle class will decrease in economic power. Decreasing in economic power means they will not be able to spend much money which will decrease the money in the economic system eventually decreasing profits and investment into the job market. The lower class will continue to decline leading to other social problems created by poverty.