If people were left to their own devices, the natural goodness of the human nature would reign and government would become irrelevant. The important question this challenge raises for Marx is why government is the best agent to improve society. It is here to facilitate the transformation to self-control and not to perpetuate itself indefinitely. This step is necessary in order to acclimatize people to a new type of society, to purge them of their previous bourgeois illusions and create a new type of socialist citizen. Remove the tethers now, as the anarchist would have it, and the result would be an egoistic chaos, a Hobbessian war of all against all.
Government controls society until it is ready to control itself.
It does not, though, answer the libertarian who would decry Marx as offensively paternalistic, violating people's rights to determine their own destiny absent government coercion. Marx would respond that it is naive to think that people control their own destiny. Eliminating explicit government intervention in one's life does not eliminate the influence of society altogether. People possess rights not to be used for broad social ends without their consent. Even if we agree with Marx's ideal society, there is value in people's coming to such a society on their own accord.