The difficulty capitalists face, however, is that this marginal difference is not constantly assured: the resources to produce the commodity will deplete, the commodity may saturate its market, or some externality (such as labour costs or pollution) will become too onerous for the commodity to maintain its advantage.
You have a misunderstanding of what externalities are. Labor costs are not an externality. Pollution can be an externality, but that by definition cannot make a commodity less profitable. Coal power plants are bad for society and the world precisely because pollution is an externality and does not reduce the profitability of coal, so the producers of electricity can earn profits by burning it and the consumers of coal power benefit from low prices but neither group suffers the cost of the pollution, which is borne by people breathing the smoggy downstream air or impacted by global warming. If there were any externality tax, then pollution would make coal less profitable, but it would no longer be bad to operate a coal power plant (we’d do it if and only if the benefits of the electricity exceeded the harms of the pollution).
That’s also not the main difficulty faced by capitalists—rather, it’s competition from other capitalists.
You have a misunderstanding of what externalities are. Labor costs are not an externality. Pollution can be an externality, but that by definition cannot make a commodity less profitable. Coal power plants are bad for society and the world precisely because pollution is an externality and does not reduce the profitability of coal, so the producers of electricity can earn profits by burning it and the consumers of coal power benefit from low prices but neither group suffers the cost of the pollution, which is borne by people breathing the smoggy downstream air or impacted by global warming. If there were any externality tax, then pollution would make coal less profitable, but it would no longer be bad to operate a coal power plant (we’d do it if and only if the benefits of the electricity exceeded the harms of the pollution).
That’s also not the main difficulty faced by capitalists—rather, it’s competition from other capitalists.