Marxists don't believe in the underconsumption theory of Keynes.

Workers wages are determined by the minimums which is required for capitalist reproduction. This varies by respective positions on the global capitalist food chain. Where Imperialist countries require the assistance of large sectors of its working class to repress and exploit their foreign comrades, local labor aristocracies develop. Middle class wages appear and consciousness is deformed.

Thus all the confusions around what is a worker and should be his or her wages. The only form of transaction which contains a sustainable level of exploitation that is transformed into profit is between those who own the means of production and those who only have their labor power to sell.

Capitalists cannot exploit other capitalists in the long run. It just turns them into monopolists.

The only real question is rather one believes that sole source of surplus is from the working class.

Whether you believe that the capitalists perform magic and produce statistically meaningful quantities of surplus. Whatever quantities of surplus they may produce they are easily able to get paid for. So this surplus is not the source of their profit.

People are constantly confusing the wages due to capitalists and their profits. They are very different things. We can easily quantify the value added in their wages. It nowhere comes close to the level of their profits.