Standard theories of production clearly distinguish between fixed and variable costs. Moreover, it is well understood that this distinction depends on the time horizon, with more costs being variable for longer horizons.
Moreover, concepts like economics of scale (with low marginal costs of producing an additional unit, as you state as an example) are well understood for certain products in certain circumstances.
The distinction between and relevance of average and marginal costs is taught in undergraduate classes.
Whether or not you can draw a nice diagram of supply and demand is pretty irrelevant for professional economists and our understanding of markets, their dynamics, and equilibria.
Moreover, concepts like economics of scale (with low marginal costs of producing an additional unit, as you state as an example) are well understood for certain products in certain circumstances.
The distinction between and relevance of average and marginal costs is taught in undergraduate classes.
Whether or not you can draw a nice diagram of supply and demand is pretty irrelevant for professional economists and our understanding of markets, their dynamics, and equilibria.