At the end of the day, Marx can never debunk Keynes, but Keynes can debunk Marx. The reason is that Marxism is a social theory and a theory of history, not an economic theory; it completely reframes the topic. Keynes posits that price and wage rigidity are the underlying elements of social cohesion and social planning, but a Marxist will simply hand-wave this away, arguing that these are merely historical forces of monopoly power that constrain people into a particular way of behaving. They argue that once price and wage rigidity collapse—due to the rate of profit reaching zero and the impossibility of market clearance—they simply will not exist in the future, and there will be perfect harmony between supply and demand based on qualitative measures. This is because quality represents use-value, while quantity is the exchange-value of a bourgeois society. Socialism is a universal qualitative construct according to Marxists, in the sense that a quality cannot become property; property relies on distinction and division, but nobody can own a quality. A quality is what existence is, and the activity of existence itself. Property is a thing that becomes the subject, is divided from its object, and owns its property by separating form from matter. Property is an acquisition of something or existence; it is reflected and gives the subject its identity, not by becoming the subjective, but by being able to reflect the subject on what it is and what it is not.
Keynes believes in sociali planning and that in motion there has to be motion preserved, which is where rigidity comes from and this motion preserved is how society organises itself, that its able to create certain conditions out of rigid prices, and for those rigid prices to have a market clearing since there isnt a market equalibrium that its the role of the government to make effective demand to clear the market. A Marxist would say that this is a control of monopolies to steal the wages and production capacity off labourers since the prices are frozen for moments in time and that within these moments value is being extracted for asocial reasons.