Capitalism still presides over regular mass slaughters by hunger which are an indictment of any social system. In the United States, the richest capitalist country in the world, thousands of people sleep on the streets, or get a living only through the drug trade. Stalinism was not an attempt to go beyond advanced capitalism on the basis of the achievements of advanced capitalism which has proved by its failure the hopelessness of all such attempts. It was an experience on the fringes of world capitalism, arising out of the defeat of a working class revolution, and stifling under its own contradictory bureaucratic regime. Stalinism was part of the pre-history humankind must grow beyond.
Socialism is discredited because only a free market economy can give a secure basis for democracy.
Without it you get state control, and state control inevitably stifles democracy. Marxists do not want any sort of bureaucratic state, neither that of a country like Britain, where the bureaucratic state works in tandem with the bourgeoisie, nor that of the Stalinist systems where the bureaucracy was the sole master of society's wealth. The Bolsheviks wanted that, too. They could not create it because of the backwardness of the isolated USSR, but it would be entirely possible in a country like Britain, especially with modern technology. It is a prize example of the crazy logic satirised by George Orwell according to which war is peace and lies are truth.