I think it helps to be able to point at AES (actually existing socialism) states, like China's. Note that China does have police, they do enforce the working class vanguard party being in control, they do punish (sometimes severely) people who abuse positions of power. Communism is not the absence of corrupt behavior and transitioning to communism is a process, not the press of a button.

That said, arguments about inevitability of human nature are usually varying degrees of historically illiterate/misinformed. People have not acted identically in every society and societal structure, neither right now across countries, nor across time.

Dialectical and historical materialism teaches us that people act in relation to the material they are immersed in, not irrespective of it. This is not to say their actions are purely mechanical, with no autonomy (they also have an inner decision-making world, which is both its own set of behaviors and also impacted by the internalizing of the external, from upbringing, culture, etc.). But what it does mean is that, for example, if you lock people in a room and say "You can't eat unless you compete with others in the room for dominance over who gets how much of the limited food and if you try to share evenly, you will lose the food," some people are going to start fighting each other, sooner or later. That's sort of an oversimplified microcosm of what capitalism can be like. People respond most viscerally to what they need and what it takes to get what they need, so when a society is built on perverse incentives and manufactured competition, people are going to be more brutal and individualistic.

As you say, transitioning away from something like this that is embedded can take generations because there is also the belief/culture/norms layer of things that exists prior to a change in society and its incentives. This is key to what marxism-leninism is about. That you can't seize power, press the communism button, and abolish all suffering overnight. So you need a vanguard who can manage the transition, without counterrevolutionary forces reversing your efforts; transitioning both from one material base of society to another and one ideas layer superstructure of society to another. Inner and outer world, to put it one way.