My understanding of your comment is that open markets means competition. Competitors will compete on price, forcing prices down, with constant production costs margins will "tend to zero"

However this is incorrect. Where demand is greater than supply, prices (and therefore profits) may be stable or rise regardless of the number of competitors. A drought is a scenario of restricted supply where all competitors in the water supply market may increase profits due to demand.

Additionally, competition in markets where there is imperfect knowledge is not always based on price. It may be based on many other factors for example branding etc.

So your statement "In open markets margins should tend to zero" is not true, they will tend towards an equilibrium based on supply and demand, and even then individual suppliers may be differentiated by factors other than price allowing them to make a higher margin than their competitors.

The fact that competition may in some cases reduce margins, does not therefore have any "wider implications on the ability to profit over the long run in a capitalist society."

Note that where the marginal cost of production is zero, we can say that supply is infinite, and so prices cannot be maintained by restricted supply. This is a different case. But I cannot think of any market where the marginal cost of production is zero. Even copying and pasting a news article requires a small investment of time. So this qualifier may be unneccessary since it refers to an absurd situation.