you see, in philosophy there is the relative concept of "validity" as opposed to "soundness".
if we examine marx's argument/logic rudimentarily - then you cannot deny that he is valid, given certain premises, i.e. that humans are not self-serving. if the premises STAND, then by his flow of logic, he is not wrong.
of course, we all know that humans are always self-serving, and the premises will never stand, thus it is not a sound argument. nonetheless, because it is valid, it can be considered to be right, only to a certain degree. i hope i'm not sounding too argumentative here, just presenting my point of view on what constitutes right and wrong.
to give an example of a ridiculous valid argument:
1. cows have wings
2. all animals with wings can fly
3. cows can fly
so if you say marx was entirely wrong, i don't think it is as simplistic as that. he was definitely overly optimistic about human behaviour though.
as for #2, there are many situations whereby economic rationality flies out of the window. just for the sake of argument, do you not agree that there are people who exist that might actually place the wellbeing of the state before self? and if we gather these people, no matter how few, and dump them onto an island where a socialist ideology, that socialism might work in that micro-community? (even though undoubtedly all the capitalist countries will seek to undermine that way of thought and life) this is a thought experiment, if you are familiar with the term, and not meant to be realistic. i hope no one comes barging in and starts complaining about how silly it sounds, because the matrix was based on another thought experiment.. and no one complained about how silly it sounded.