> Marx's basic argument in Capital is that labor is what creates value in capital goods.
Capital is an economic work which is of minor relevance to his political ideologies. It tries to explain and provide theories that certainly would support some of his political views if true, but Capital is not a pre-requisite for the political ideology (in as much as there's a large number of other possible theories that could equally provide justifications for the political ideology).
> If there is no exploitation you don't have Marxism in the first place
That's simply not true at all. For the political ideology, the concept of exploitation is merely one of many arguments used to justify why the working class should consider it morally acceptable to overthrow the capitalist regime. It was realpolitik.
It's worth noting that Marx' philosophical works are far more "capitalist friendly" than most modern day socialists, for example. Marx may have talked about exploitation, but he also talked about these structures as equally binding the capitalist into a role he could not escape, and spoke with admiration about the development the growth of capitalism was creating. After all, according to Marx, the growth of capitalism is what will make socialism possible. But those bits don't get people out in the streets. Talk of exploitation does. And so Marx-the-politician was far more aggressive in terms of language than Marx-the-economist or Marx-the-philosopher.
The subjective view of the working classes on whether or not there is exploitation is the only thing that ultimately matters in the context of his political ideology, and even then only because it has historically been an effective recruitment factor.
Other than that, the presence or absence of exploitation is relatively irrelevant to Marxism. Marx ideas about the structural development of social and economic systems and inevitability of socialism, for example, does not rest on exploitation, but on whether or not capitalism eventually will develop to a state where it causes sufficient social upheaval to be a catalyst for new revolutionary movements amongst the working classes, and whether or not the structure of this will lead to a socialist system.