Do you not consider it a serious blow of any kind that Marx's concrete economic theories are fundamentally incorrect, and as a result, his economic prescriptions have no basis in reality? How does the capital class steal from the labor class if labor is not the only source of value?
Surplus value theory. It states that labor produces all the economic surplus assuming LTV is true. Since we assume LTV IS true, capital leeches off of labor and we would be collectively better off without capital if they were removed by communal ownership of the means of production.
Let’s say that a capitalist employs a laborer to build a chair and pays him $10 to do so; the capitalist provides the tools of course. After that, the capitalist sells the chair for $40. Marx would look at that and say that the capitalist has captured $30 of the value of that chair unrightfully because the only source of value is labor. If LTV, goes out the window, then this falls apart.
Maybe the capitalist provides value by taking on the risk of owning the business which will be subject to market competition. The laborer will be paid his wage regardless of market conditions for chairs and is thus not subject to the same risk. The capitalist adds value in organizing bringing the materials to a place that the laborer can construct the chair. The capitalist controls the time dimension of the investment. He considers whether he would rather spend more time on a higher priced chair or less time on a lower priced chair. The capitalist makes decisions about his investments in fixed capital as in what quality of tools, what kinds of tools, etc are provided to the chairmaker.