The theory is that socialism will emerge from capitalism, Marx made this claim in Critique of the Gotha Programme.
Marxism, or a bastardized verision thereof has failed each and every time it's been attempted.
The nature of man? If by that you mean that how we organise ourselves now is somehow guided by our humaneness? I've heard this argument before, that capitalism reflects the true nature of humanity. It's not a good argument. I mentioned other forms of economic system, including slavery, but even before that humans banded together for mutual advantage – the nature of humans is to cooperate for survival, not to compete with one another inside a micro society. The competition in capitalism is a human construct, it's an invention, it isn't innate to human nature. We only think it is because we've known nothing else.
I'm making no such argument as yours, so your response to whatever argument you have heard before isn't relevant.
The nature of humans is to survive, not to cooperate for survival. If survival means taking what another human has, then one either succeeds or fails in that endeavor.
Man is an animal. It is perfectly normal animal behavior to compete for survivial. Put man in a society and the strong will always compete to survive "better." I completely disagree it is a "human" construct. Natural selection is part of biology and observed in almost every animal species on Earth.
The point of Marx's argument about the emergence of socialism was that eventually the state and its appartus disappear, but that early socialism is, in his phrase, “still stamped with the birthmarks of the old society from whose womb it emerges.” The examples we've seen thus far have been stillborn.
And again, it has no chance of succeeding as long as Man is involved in it. Where it WILL succeed is in a flock of sheep. That's because they have no instinct for survival nor a desire that extends beyond mere existence at the moment.
Your points about people who go out and earn more should have more being fair; redistribution being punishment and stifling the urge to excel are factors related to capitalism, they're not critiques of socialism.
You're absolutely correct because there is nothing fair about socialism. It forces the strong to prop up the weak.
Man's progress doesn't have much to do with the desire to excel, it's probably more about survival. If you take a quick look at human pre-history to now you can see that humans only got to this point because of their ability to cooperate to survive. As for utopia, I've never said that there would be a utopia, just a lack of exploitation. I would think that factor alone would improve the level of happiness in humanity.
Of course Man's progress has to do with a desire to excel. If it was only about survival we'd still live in caves according to your logic. But the desire to survive better is a desire to excel. That's brought us from the cave to the condo. And those who excelled throughout history -- the inventors -- worked as individuals, not as members of a community to do so. In fact, in most documented cases, progress had ot drag the community kicking and screaming to new ideals.
Socialism amounts to being the aforementioned flock of sheep. That is not living, IMO. That's existing, waiting to die. And again, you will never achieve true socialism with Man because those who wish to excel and be rewarded for doing so will never accept it. I sure as Hell won't.