surplus theory?

Opposing to Marx' theory, economists like Robinson more or less state that:
capital also contributes to creating surplus-value by making the means of production available...

Do they mean that the buyers of labor use profit to buy better machines -> produce more and consequently can be in the advantage of both?

The capitalists own both the labor power of the proletarians they employ, and the means of production (machinery, tools) that they employ them to operate already. But it is important to remember that the original production of the tools/machinery would be impossible without the original labor to make them. This labor is of course performed by proletarians.

It's hard to tell from one line, but your explanation sounds likely. It's a common rebuttal to marxist ideas if they even go as far to admit the role of labor in creating (at least in part) wealth - many mainstream economists simply argue that investment creates wealth and labor is just one aspect of the means and adds nothing. So yeah, there's an argument that labor creates wealth, but so does the machinery and factory which are owned/provided by the capitalist. Of course then the response is where did that wealth come from to create the machines and factories in the first place if not labor at some prior point?

Also remember the means of production were centralized in the hands of the capitalist by seperating workers from the land (e.g., serfs/slaves) and seperating them from the means of production (e.g., tools - like a local blacksmith), giving them no choice but to create surplus value for a capitalist if they wanted to survive. See "primative accumulation."

Actually, many of them preferred vagabondage over wage slavery but vagabondage was punished by death, leaving only the option wage slavery or death. So the capitalist doesn't provide means of production for the laborer, he steals it and gives the laborer no other choice but death.

Are you implying with writing in present tense that this is still the case?
The wage-laborers now, at least in my country, includes 90% of the population (widely spoken). The 'but' is: they are a very heterogenous group etc. They don't really have common interests.

How do they not have common interests? Maybe they pursue different interests which they base on cultural, or specialized labor interests, but that does not mean that the common interests aren't still there. If 90% of your population must sell their labor power as a commodity to subsist, then it is in their interests to abolish the system which necessitates this exploitative exchange, regardless of whether the population is conscious of this or not.

Well, I also included officials etc.
They never formed a common front against capital. The manual working class did once though.
The poverty reduced very, very much, under centre-parties. People can develop a lot here, thanks to free speech/free morals etc.
However, some centre-right parties close their eyes to side of capitalism too much, definitely

I am not familiar with the Netherlands' economic history but if there was dire poverty once, but there isn't any longer than it is reasonable to assume that much of the exploitation of your countries workers have been exported to third world countries. This is how imperialism works, advanced capitalist countries pacify their own working class by hyper-exploiting the population of other, less developed countries.

This is unsustainable, as the workers of hyper-exploited "colonies" if you will, will carry on their own struggles forcing concessions from the capitalists, and resulting in the rebalancing out of exploitation.

The problem with wage-labour is not that it is actually heterogenous; it is clear, using Marxist analysis, who is exploited and who exploits. But culturally and socially, many workers are wage-labourers who are given a nominal salary, a 'manager'-sounding position (you know, supervisor roles and so on) and so on. Aspiration also divides the working class - between those who have just enough money to hope to aspire to riches [yet ultimately fall short a la the caricature of lower-middle class England, Basil Fawlty] and those who have too little money to aspire, the 'traditional' working class if you will. This aspiration gap has definitely led, over the course of centuries, to an entrenched social and cultural divide, to the extent that the working class itself is split between those seen as 'chavs' and those seen as 'middle class', even though neither epithet is either warranted nor an accurate socio-economic descriptor.

In response to the OP: I actually find Marx's critique of Capitalism and the identification of surplus' exploitative properties to be one of the least controversial of all of Marxism's ideas. Even many Capitalists agree with this critique, but obviously take a 'this is real life' or 'job creators' attitude towards the bourgeoisie.

Thanks for your good post :)

What do you mean with the 'this is real life' attitude?

Like, when you argue with people who are pro-Capitalist and when they've exhausted all logic they'll just be like 'well, that's just the way it is', or 'this is how life is', that sort of thing.

I don't think they mean that.
It's an old argument which boils down to a very simple statement: without the means of production no labour can achieve anything, but the real twist is that these people assume that the means of production can only be "made available" in the form of capital, effectively meaning that labour cannot exist without capital. It is not a more or less scientific explanation of the whole process, but rather a sort of a moralist defense of the practice of exploitation. It doesn't explain how surplus value is produced, but merely justifies profit by assuming that no means of production could be put to use if it didn't appear in the form of capital.
In other words, this statement is obviously banal and tautological, but it hides some other aspects of capitalist apologism.