Self interest is the natural incentive that makes survival possible. Politics works on the belief that there is also an incentive to serve. The natural incentive to serve exists only in the imagination. Voters go to the poles believing that those running for office have every intention of serving them selflessly. This one misunderstanding is the root of all that goes wrong where government is concerned.
If you ever want to debunk an economic theory, notice whether or not the outcome depends upon a human being acting selflessly. In Keynes theory, he assumes a straw man. The straw man is his version of a selfless entity. Selfless humans do not exist and any theory that assumes the existence of one or more of them is not going to work.
Even mother Teresa acted out of self interest. Chances are her aspect of acting selflessly was acquired. Helping others truly gave her satisfaction. Mother Teresa would have never run for office. It is doubtful she would have any interest in governing other people.